Cambodian campaign

The Cambodian campaign (also known as the Cambodian incursion and the Cambodian invasion) was a series of military operations conducted in eastern Cambodia, a neutral country, during 1970 by the United States and the Republic of Vietnam (South Vietnam) as an extension of the Vietnam War and the Cambodian Civil War. The invasions were a policy of President Richard Nixon; 13 major operations were conducted by the Army of the Republic of Vietnam (ARVN) between 29 April and 22 July and by US forces between 1 May and 30 June.

The objective of the campaign was the defeat of the approximately 40,000 troops of the People's Army of Vietnam (PAVN) and the Viet Cong (VC) in the eastern border regions of Cambodia. Cambodian neutrality and military weakness made its territory a safe zone where Vietnamese communist forces could establish bases for operations over the border. With the US shifting toward a policy of Vietnamization and withdrawal, it sought to shore up the South Vietnamese government by eliminating the cross-border threat.

A change in the Cambodian government allowed an opportunity to destroy the bases in 1970, when Prince Norodom Sihanouk was deposed and replaced by pro-US General Lon Nol. A series of South Vietnamese-Lon Nol operations had captured a few towns, but the Viet Cong military and political leadership had narrowly escaped the cordon. The operation was partly a response to a North Vietnamese offensive on 29 March against the Cambodian Army that captured large parts of eastern Cambodia in the wake of these operations. Allied military operations failed to eliminate many communist troops or to capture their elusive headquarters, known as the Central Office for South Vietnam (COSVN) as they had left a month prior, but the haul of captured material in Cambodia prompted claims of success.

Background
The PAVN had been utilizing large sections of relatively unpopulated eastern Cambodia as sanctuaries into which they could withdraw from the struggle in South Vietnam to rest and reorganize without being attacked. These base areas were also utilized by the PAVN and VC to store weapons and other material that had been transported on a large scale into the region on the Sihanouk Trail. PAVN forces had begun moving through Cambodian territory as early as 1963. Cambodian neutrality had already been violated by South Vietnamese forces in pursuit of political-military factions opposed to the regime of Ngô Đình Diệm in the late 1950s and early 1960s. In 1966, Prince Norodom Sihanouk, ruler of Cambodia, convinced of eventual communist victory in Southeast Asia and fearful for the future of his rule, had concluded an agreement with the People's Republic of China which allowed the establishment of permanent communist bases on Cambodian soil and the use of the Cambodian port of Sihanoukville for resupply.

During 1968, Cambodia's indigenous communist movement, labeled Khmer Rouge (Red Khmers) by Sihanouk, began an insurgency to overthrow the government. While they received very limited material help from the North Vietnamese at the time (the Hanoi government had no incentive to overthrow Sihanouk, since it was satisfied with his continued "neutrality"), they were able to shelter their forces in areas controlled by PAVN/VC troops.

The US government was aware of these activities in Cambodia, but refrained from taking overt military action within Cambodia in hopes of convincing the mercurial Sihanouk to alter his position. To accomplish this, President Lyndon B. Johnson authorized covert cross-border reconnaissance operations conducted by the secret Studies and Observations Group in order to gather intelligence on PAVN/VC activities in the border regions (Project Vesuvius). This intelligence data would then be presented to the prince in an effort to change his mind.

Menu, coup and North Vietnamese offensive
The new commander of the US Military Assistance Command, Vietnam (MACV), General Creighton W. Abrams, recommended to President Richard M. Nixon shortly after his inauguration that the Cambodian Base Areas be attacked by aerial bombardment utilizing B-52 Stratofortress bombers. The president initially refused, but the breaking point came with the launching of PAVN's Tet 1969 Offensive within South Vietnam. Nixon, angered at what he perceived as a violation of the "agreement" with Hanoi after the cessation of the bombing of North Vietnam, authorized the covert air campaign. The first mission of Operation Menu was dispatched on 18 March and by the time it was completed 14 months later more than 3,000 sorties had been flown and 108,000 tons of ordnance had been dropped on eastern Cambodia.

While Sihanouk was abroad in France for a rest cure in January 1970, government-sponsored anti-Vietnamese demonstrations were held throughout Cambodia. Continued unrest spurred Prime Minister/Defense Minister Lon Nol to close the port of Sihanoukville to communist supplies and to issue an ultimatum on 12 March to the North Vietnamese to withdraw their forces from Cambodia within 72 hours. The prince, outraged that his "modus vivendi" with the communists had been disturbed, immediately arranged for a trip to Moscow and Beijing in an attempt to gain their agreement to apply pressure on Hanoi to restrain its forces in Cambodia.

On 18 March, the Cambodian National Assembly deposed Sihanouk and named Lon Nol as provisional head of state. This led Sihanouk to immediately establish a government-in-exile in Beijing and to ally himself with North Vietnam, the Khmer Rouge, the VC, and the Laotian Pathet Lao. In doing so, Sihanouk lent his name and popularity in the rural areas of Cambodia to a movement over which he had little control. The North Vietnamese response to the coup was swift. PAVN began directly supplying large amounts of weapons and advisors to the Khmer Rouge, and Cambodia plunged into civil war.

Lon Nol saw Cambodia's population of 400,000 ethnic Vietnamese as possible hostages to prevent PAVN attacks and ordered their roundup and internment. Cambodian soldiers and civilians then unleashed a reign of terror, murdering thousands of Vietnamese civilians. On 15 April for example, 800 Vietnamese men had been rounded up at the village of Churi Changwar, tied together, executed, and their bodies dumped into the Mekong River. They then floated downstream into South Vietnam. Cambodia's actions were denounced by both the North and South Vietnamese governments.

Even before the supply conduit through Sihanoukville was shut down, PAVN had begun expanding its logistical system from southeastern Laos (the Ho Chi Minh trail) into northeastern Cambodia. PAVN also launched an offensive (Campaign X) against the Cambodian Khmer National Armed Forces (FANK), quickly seizing large portions of the eastern and northeastern parts of the country, isolating and besieging or overrunning a number of Cambodian cities including Kampong Cham. Communist forces then approached within 20 mi of the capital, Phnom Penh, spurring President Nixon into action.

On 29 March 1970, the PAVN had taken matters into their own hands and launched an offensive against the FANK with documents uncovered from the Soviet archives revealing that the offensive was launched at the explicit request of the Khmer Rouge following negotiations with Nuon Chea. A PAVN force quickly overran large parts of eastern Cambodia reaching to within 15 mi of Phnom Penh. After defeating those forces, the PAVN turned the newly won territories over to the local insurgents. The Khmer Rouge also established "liberated" areas in the south and the southwestern parts of the country, where they operated independently of the North Vietnamese.

Planning
In response to events in Cambodia, President Nixon believed that there were distinct possibilities for a U.S. response. With Sihanouk gone, conditions were ripe for strong measures against the Base Areas. He was also adamant that some action be taken to support "The only government in Cambodia in the last twenty-five years that had the guts to take a pro-Western stand. The president then solicited proposals for actions from the Joint Chiefs of Staff and MACV, who presented him with a series of options: a naval quarantine of the Cambodian coast; the launching of South Vietnamese and American airstrikes; the expansion of hot pursuit across the border by ARVN forces; or a ground invasion by ARVN, U.S. forces, or both.

During a televised address on 20 April, Nixon announced the withdrawal of 150,000 U.S. troops from South Vietnam during the year. This planned withdrawal implied restrictions on any offensive U.S. action in Cambodia. By the spring of 1970, MACV still maintained 330,648 U.S. Army and 55,039 Marine Corps troops in South Vietnam, most of whom were concentrated in 81 infantry and tank battalions. Many of them, however, were preparing to leave the country or expected to leave in the near future and would not be available for immediate combat operations.

On 22 April Nixon authorized the planning of a South Vietnamese incursion into the Parrot's Beak (named for its perceived shape on a map), believing that "Giving the South Vietnamese an operation of their own would be a major boost to their morale as well as provide a practical demonstration of the success of Vietnamization." On the following day, Secretary of State William P. Rogers testified before the House Appropriations Subcommittee that "the administration had no intentions...to escalate the war. We recognize that if we escalate and get involved in Cambodia with our ground troops that our whole program [Vietnamization] is defeated."

South Vietnamese forces had been rehearsing for just such an operation since late March. On 27 April, an ARVN Ranger Battalion had advanced into Kandal Province to destroy a communist base. Four days later other South Vietnamese troops drove 16 kilometers into Cambodian territory. Lon Nol, who had initially attempted to follow a neutralist policy of his own, requested military aid and assistance from the U.S. government on 14 April. On that day, South Vietnamese forces then conducted the first of three brief cross-border operations under the aegis of Operation Toan Thang (Complete Victory) 41, sending armored cavalry units into regions of Cambodia's Svay Rieng Province nicknamed the Angel's Wing and the Crow's Nest. On 20 April, 2,000 South Vietnamese troops advanced into the Parrot's Beak, killing 144 PAVN troops. On 22 April, Nixon authorized American air support for the South Vietnamese operations. All of these incursions into Cambodian territory were simply reconnaissance missions in preparation for a larger-scale effort being planned by MACV and its ARVN counterparts, subject to authorization by Nixon.

President Nixon then authorized General Abrams to begin planning for a U.S. operation in the Fishhook region. A preliminary operational plan had actually been completed in March, but was kept so tightly under wraps that when Abrams handed over the task to General Michael Davison, commander of the II Field Force, he was not informed about the previous planning and started a new one from scratch. Seventy-two hours later, Davison's plan was submitted to the White House. National Security Advisor Henry Kissinger asked one of his aides to review it on 26 April, and the NSC staffer was appalled by its "sloppiness".

The main problems were the pressure of time and the desire of the U.S. president for secrecy. The Cambodian monsoon, whose heavy rains would hamper operations, was only two months away. By the order of the president, the State Department did not notify the Cambodian desk at the US Embassy, Saigon, the Phnom Penh embassy, or Lon Nol of the planning. Operational security was as tight as General Abrams could make it. There was to be no prior U.S. logistical build-up in the border regions which might serve as a signal to the communists. U.S. brigade commanders were informed only a week in advance of the offensive, while battalion commanders got only two or three days' notice.

Decisions
Not all of the members of the administration agreed that an invasion of Cambodia was either militarily or politically expedient. Secretary of Defense Melvin R. Laird and Secretary Rogers were both opposed to any such operation due to their belief that it would engender intense domestic opposition in the U.S. and that it might possibly derail the ongoing peace negotiations in Paris (they had both opposed the Menu bombings for the same reasons). Both were castigated by Henry Kissinger for their "bureaucratic foot-dragging." As a result, Laird was bypassed by the Joint Chiefs in advising the White House on planning and preparations for the Cambodian operation.



On the evening of 25 April Nixon dined with his friend Bebe Rebozo and Kissinger. Afterward, they screened one of Nixon's favorite movies, Patton, a biographical portrayal of controversial General George S. Patton, Jr., which he had seen five times previously. Kissinger later commented that "When he was pressed to the wall, his [Nixon's] romantic streak surfaced and he would see himself as a beleaguered military commander in the tradition of Patton.

The following evening, Nixon decided that "We would go for broke" and gave his authorization for the incursion. The joint U.S./ARVN campaign would begin on 1 May with the stated goals of: reducing allied casualties in South Vietnam; assuring the continued withdrawal of U.S. forces; and enhancing the U.S./Saigon government position at the peace negotiations in Paris.

In order to keep the campaign as low-key as possible, General Abrams had suggested that the commencement of the incursion be routinely announced from Saigon. At 21:00 on 30 April, however, President Nixon appeared on all three U.S. television networks to announce that "It is not our power but our will and character that is being tested tonight" and that "the time has come for action." He announced his decision to launch American forces into Cambodia with the special objective of capturing COSVN, "the headquarters of the entire communist military operation in South Vietnam." COSVN as a single headquarters for control of PAVN operations in South Vietnam probably did not exist, or, at least, was never found.

Escape of the Provisional Revolutionary Government
Planning for any eventuality the North Vietnamese started planning emergency evacuation routes in the event of a coordinated assault by Cambodians from the west and South Vietnamese from the east. After the Cambodian coup, COSVN was evacuated on 19 March 1970. While the PRG and PAVN/VC bases were preparing to also move to the north and safety they came under aerial bombardment from B-52 bombers on 27 March. As laid out by the evacuation plans General Hoàng Văn Thái planned to have three Divisions to cover the escape. The 9th Division would block any movement from the ARVN, the VC 5th Division would screen any FANK forces and the 7th Division would provide security to the civilian and military members of the PAVN/VC bases.

Moving across the border in Cambodia on 30 March elements of the PRG and VC were surrounded in their bunkers by ARVN forces flown in by helicopter. Surrounded they awaited till nightfall and then with security provided by the 7th Division they broke out of the encirclement and fled north to unite with the COSVN in the Cambodian Kratie province in what would come to be known as the Escape of the Provisional Revolutionary Government. Trương Như Tảng was the Minister of Justice in the PRG and he recounts that during the march to the northern bases was day after day of forced marches broken up by B-52 bombing raids.

Years later Trương would recall just how "close [South Vietnamese] were to annihilating or capturing the core of the Southern resistance – elite units of our frontline fighters along with the civilian and much of the military leadership". After many days of hard marches the PRG reached the northern bases, and relative safety, in the Kratie region. Casualties were light and the march even saw the birth of a baby to Dương Quỳnh Hoa, the deputy minister of health in the PRG. The column needed many days to recover and Trương himself would require weeks to recover from the long march.

Parrot's Beak
South Vietnamese forces had already crossed the border on 30 April, launching Operation Toan Thang 42 (Total Victory), also labeled Operation Rock Crusher. 12 ARVN battalions of approximately 8,700 troops (two armored cavalry squadrons from III Corps and two from the 25th Division and 5th Infantry Divisions, an infantry regiment from the 25th Infantry Division, and three Ranger battalions and an attached ARVN Armored Cavalry Regt from the 3rd Ranger Group) crossed into the Parrot's Beak region of Svay Rieng Province. The offensive was under the command of Lieutenant General Đỗ Cao Trí, the commander of III Corps, who had a reputation as one of the most aggressive and competent ARVN generals. Tri's operation was to have begun on the 29th but the general refused to budge, claiming that his astrologer had told him "the heavens were not auspicious". During their first two days in Cambodia, ARVN units had several sharp encounters with PAVN forces. The PAVN, forewarned by previous ARVN incursions, however, conducted only delaying actions in order to allow the bulk of their forces to escape to the west.

The ARVN operation soon settled down to become a search and destroy mission, with troops combing the countryside in small patrols looking for PAVN supply caches. Phase II of the operation began with the arrival of elements of the 9th Infantry Division. Four tank-infantry task forces attacked into the Parrot's Beak from the south. After three days of operations, ARVN claimed 1,010 PAVN troops had been killed and 204 prisoners taken for the loss of 66 ARVN dead and 330 wounded.

Fishhook
On 1 May an even larger operation, in parallel with Toan Thang 42, known by the ARVN as Operation Toan Thang 43 and by MACV as Operation Rock Crusher, got underway as 36 B-52s dropped 774 tons of bombs along the southern edge of the Fishhook. This was followed by an hour of massed artillery fire and another hour of strikes by tactical fighter-bombers. At 10:00, the 1st Air Cavalry Division, the 11th Armored Cavalry Regiment, the 1st ARVN Armoured Cavalry Regiment, and the 3rd ARVN Airborne Brigade then entered Kampong Cham Province of Cambodia. Known as Task Force Shoemaker (after General Robert M. Shoemaker, the Assistant Division Commander of the 1st Cavalry Division), the force attacked the long-time communist stronghold with 10,000 U.S. and 5,000 South Vietnamese troops. The operation utilized mechanized infantry and armored units to drive deep into the province where they would then link up with ARVN airborne and U.S. airmobile units that had been lifted in by helicopter.

Opposition to the incursion was expected to be heavy, but PAVN/VC forces had begun moving westward two days before the advance began. By 3 May, MACV reported only eight Americans killed and 32 wounded, low casualties for such a large operation. There was only scattered and sporadic contact with delaying forces such as that experienced by elements of the U.S. 11th Armoured Cavalry three kilometers inside Cambodia. PAVN troops opened fire with small arms and rockets only to be blasted by tank fire and tactical airstrikes. When the smoke had cleared, 50 dead PAVN soldiers were counted on the battlefield while only two U.S. troops were killed during the action.

1st Battalion/7th Cavalry, was in the Fishhook very early May through 30 June when they crossed the river back into Vietnam. There was extremely heavy combat throughout the period. American losses were very heavy, with all units relying on heavy inflow of replacements to try to maintain at least half strength in the field. In one company, of all the men who had entered Cambodia, only nine left on 30 June, the rest having been either killed or wounded and evacuated. The unit was awarded the Valorous Unit Award, equivalent to individual Silver Stars, for their combat performance in the Fishhook.

The North Vietnamese had ample notice of the impending attack. A 17 March directive from the headquarters of the B-3 Front, captured during the incursion, ordered PAVN/VC forces to "break away and avoid shooting back...Our purpose is to conserve forces as much as we can". The only surprised party amongst the participants in the incursion seemed to be Lon Nol, who had been informed by neither Washington nor Saigon concerning the impending invasion of his country. He only discovered the fact after a telephone conversation with the head of the U.S. mission, who had found out about it himself from a radio broadcast.

The only conventional battle fought by American troops occurred on 1 May at the town of Snoul, the terminus of the Sihanouk Trail at the junction of Routes 7, 13, and 131. Elements of the U.S. 11th Armored Cavalry and supporting helicopters came under PAVN fire while approaching the town and its airfield. When a massed American attack was met by heavy resistance, the Americans backed off, called in air support and blasted the town for two days, reducing it to rubble. During the action, Brigadier General Donn A. Starry, commander of the 11th Armored Cavalry, was wounded by grenade fragments and evacuated.

Hunting supply caches
On the following day, Company C, 1st Battalion (Airmobile)/5th Cavalry, entered what came to be known as "The City", southwest of Snoul. The two-square mile PAVN complex contained over 400 thatched huts, storage sheds, and bunkers, each of which was packed with food, weapons, and ammunition. There were truck repair facilities, hospitals, a lumber yard, 18 mess halls, a pig farm, and even a swimming pool. Forty kilometers to the northeast, other 1st Cavalry Division elements discovered a larger base on 6 May. Nicknamed "Rock Island East" after the U.S. Army's Rock Island Arsenal in Illinois, the area contained more than 6.5 million rounds of anti-aircraft ammunition, 500,000 rifle rounds, thousands of rockets, several General Motors trucks, and large quantities of communications equipment.

The one thing that was not found was COSVN. On 1 May a tape of Nixon's announcement of the incursion was played for General Abrams, "who must have cringed" when he heard the president state that the capture of the headquarters was one of the major objectives of the operation. MACV intelligence knew that the mobile and widely dispersed headquarters would be difficult to locate. In response to a White House query before the fact, MACV had replied that "major COSVN elements are dispersed over approximately 110 square kilometers of jungle" and that "the feasibility of capturing major elements appears remote".

After the first week of operations, additional battalion and brigade units were committed to the operation, so that between 6 and 24 May, a total of 90,000 Allied troops (including 33 U.S. maneuver battalions) were conducting operations inside Cambodia. Due to increasing political and domestic turbulence in the U.S., President Nixon issued a directive on 7 May limiting the distance and duration of U.S. operations to a depth of 30 km and setting a deadline of 30 June for the withdrawal of all U.S. forces to South Vietnam.

South Vietnamese forces were not constrained by the time and geographic limitations placed upon U.S. units. From the provincial capital of Svay Rieng, ARVN elements pressed westward to Kampong Trabek, where on 14 May their 8th and 15th Armored Cavalry regiments defeated the 88th PAVN Infantry Regiment. On 23 May, the South Vietnamese pushed beyond the deepest U.S. penetrations and attacked the town of Krek.

Binh Tay and Cuu Long
In the II Corps area, Operation Binh Tay I (Operation Tame the West) was launched by the 1st and 2nd Brigades of the U.S. 4th Infantry Division and the 40th ARVN Infantry Regiment against Base Area 702 (the traditional headquarters of the PAVN B-2 Front) in northeastern Cambodia from 5–25 May. Following airstrikes, the initial American forces, assaulting via helicopter, were driven back by intense anti-aircraft fire. On the following day, the 3rd Battalion, 506th Infantry (on loan from the 101st Airborne Division), landed without opposition. Its sister unit, the 1st Battalion, 14th Infantry was also unopposed. The 3rd Battalion, 8th Infantry, however, inserted only 60 men before intense PAVN fire (which shot down one helicopter and damaged two others) shut down the landing zone, leaving them stranded and surrounded overnight. By the following morning, PAVN forces had left the area.

On the 7th, the division's 2nd Brigade inserted its three battalions unopposed. After ten days (and only one significant firefight) the American troops returned to South Vietnam, leaving the area to the ARVN. Historian Shelby Stanton has noted that "there was a noted lack of aggressiveness" in the combat assault and that the division seemed to be "suffering from almost total combat paralysis." During Operation Binh Tay II, the ARVN 22nd Division moved against Base Area 702 from 14–26 May. The second phase of the operation was carried out by ARVN forces against Base Area 701 between 20 May and 27 June when elements of the ARVN 22nd Division conducted operations against Base Area 740.

On 10 May, Bravo Company, 3rd Battalion, 506th Infantry, was ambushed by a much larger PAVN force in the Se San Valley. Eight U.S. soldiers were killed and 28 wounded. Among the killed was Spc. Leslie Sabo, Jr. (posthumously promoted to sergeant), who was recommended for the Medal of Honor, but the paperwork went missing until 1999. Sabo was awarded the Medal of Honor on 16 May 2012 by President Barack Obama.

In the III Corps Tactical Zone, Operation Toan Thang 44 (Operation Bold Lancer), was conducted by the 1st and 2nd Brigades of the 25th Infantry Division between 6 May and 30 June. The targets of the operation were Base Areas 353, 354, and 707 located north and northeast of Tay Ninh, South Vietnam. Once again, a hunt for COSVN units was conducted, this time around the Cambodian town of Memot and, once again, the search was futile. During its operations, the 25th Infantry killed 1,017 PAVN/VC troops while losing 119 of its own men killed.

Simultaneous with the launching of Toan Thang 44, the two battalions of the 3rd Brigade, 9th Infantry Division, crossed the border 48 kilometers southwest of the Fishhook into an area known as the Dog's Face from 7 through 12 May. The only significant contact with PAVN forces took place near the hamlet of Chantrea, where 51 PAVN were killed and another 21 were captured. During the operation, the brigade lost eight men killed and 22 wounded. It was already too late for thousands of ethnic Vietnamese murdered by Cambodian persecution, but there were tens of thousands of Vietnamese still within the country who could now be evacuated to safety. South Vietnamese President Nguyễn Văn Thiệu arranged with Lon Nol to repatriate as many as were willing to leave. The new relationship did not, however, prevent the Cambodian government from stripping the Vietnamese of their homes and other personal property before they left.

Thiệu then authorized Operation Cuu Long, in which ARVN ground forces, including mechanized and armored units, drove west and northwest up the eastern side of the Mekong River from 9 May – 1 July. A combined force of 110 Republic of Vietnam Navy and 30 U.S. vessels proceeded up the Mekong to Prey Veng, permitting IV Corps ground forces to move westward to Phnom Penh and to aid ethnic Vietnamese seeking flight to South Vietnam. During these operations South Vietnamese and American naval forces evacuated about 35,000 Vietnamese from Cambodia. Those who did not wish to be repatriated were then forcibly expelled. Surprisingly, North Vietnamese forces did not oppose the evacuation, though they could easily have done so.

Other operations conducted from IV Corps included Operation Cuu Long II (16–24 May), which continued actions along the western side of the Mekong. Lon Nol had requested that the ARVN help in the retaking of Kompong Speu, a town along Route 4 southwest of Phnom Penh and 90 mi inside Cambodia. A 4,000-man ARVN armoured task force linked up with Cambodian ground troops and then retook the town. Operation Cuu Long III (24 May – 30 June) was an evolution of the previous operations after U.S. forces had left Cambodia.

After rescuing the Vietnamese from the Cambodians, ARVN was tasked with saving the Cambodians from the North Vietnamese. The goal was to relieve the city of Kompong Cham, 70 kilometers northwest of the capital and the site of the headquarters of Cambodia's Military Region I. On 23 May, General Trí led a column of 10,000 ARVN troops along Route 7 to the 180 acre Chup rubber plantation, where PAVN resistance was expected to be heavy. Surprisingly, no battle ensued and the siege of Kompong Cham was lifted at a cost of 98 PAVN troops killed.

Air support and logistics
Aerial operations for the incursion got off to a slow start. Reconnaissance flights over the operational area were restricted since MACV believed that they might serve as a signal of intention. The role of the Air Force in the planning for the incursion itself was minimal at best, in part to preserve the secrecy of Menu which was then considered an overture to the thrust across the border.

On 17 April, General Abrams requested that the president approve Operation Patio, covert tactical airstrikes in support of Studies and Observations Group recon elements "across the fence" in Cambodia. This authorization was given, allowing U.S. aircraft to penetrate 13 mi into northeastern Cambodia. This boundary was extended to 29 mi along the entire frontier on 25 April. Patio was terminated on 18 May after 156 sorties had been flown. The last Menu mission was flown on 26 May.

During the incursion itself, U.S. and ARVN ground units were supported by 9,878 aerial sorties (6,012 U.S./2,966 Vietnamese Air Force), an average of 210 per day. During operations in the Fishhook, for example, the USAF flew 3,047 sorties and the Republic of Vietnam Air Force 332. These tactical airstrikes were supplemented by 653 B-52 missions in the border regions (71 supporting Binh Tay operations, 559 for Toan Thang operations, and 23 for Cuu Long).

30 May saw the inauguration of Operation Freedom Deal (named as of 6 June), a continuous U.S. aerial interdiction campaign conducted in Cambodia. These missions were limited to a depth of 48-kilometers between the South Vietnamese border and the Mekong River. Within two months, however, the limit of the operational area was extended past the Mekong, and U.S. tactical aircraft were soon directly supporting Cambodian forces in the field. These missions were officially denied by the U.S. and false coordinates were given in official reports to hide their existence. Defense Department records indicated that out of more than 8,000 combat sorties flown in Cambodia between July 1970 and February 1971, approximately 40 percent were flown outside the authorized boundary.

The real struggle for the U.S. and ARVN forces in Cambodia was the effort at keeping their units supplied. Once again, the need for security before the operations and the rapidity with which units were transferred to the border regions precluded detailed planning and preparation. Abrams was fortunate—had PAVN fought for the sanctuaries instead of fleeing, U.S. and ARVN units would have rapidly consumed their available supplies. This situation was exacerbated by the poor road network in the border regions and the possibility of ambush for nighttime road convoys demanded that deliveries only take place during daylight. The tempo of logistical troops could be mind numbing. The U.S. Third Ordnance Battalion for example, loaded up to 150 flatbed trucks per day with ammunition. Logisticians were issuing more than 2,300 short tons (almost five million pounds) of supplies every day to support the incursion. Aerial resupply, therefore, became the chief method of logistical replenishment for the forward units. Military engineers and aviators were kept in constant motion throughout the incursion zone.

Due to the rapid pace of operations, deployment, and redeployment, coordination of artillery units and their fires became a worrisome quandary during the operations. This was made even more problematic by the confusion generated by the lack of adequate communications systems between the rapidly advancing units. The joint nature of the operation added another level of complexity to the already overstretched communications network. Regardless, due to the ability of U.S. logisticians to innovate and improvise, supplies of food, water, ammunition, and spare parts arrived at their destinations without any shortages hampering combat operations and the communications system, although complicated, functioned well enough during the short duration of U.S. operations.

Aftermath
The North Vietnamese response to the incursion was to avoid contact with allied forces and, if possible, to fall back westward and regroup. PAVN forces were well aware of the planned attack and many COSVN/B-3 Front military units were already far to the north and west conducting operations against the Cambodians when the offensive began. During 1969 PAVN logistical units had already begun the largest expansion of the Ho Chi Minh trail conducted during the entire conflict. As a response to the loss of their Cambodian supply route, North Vietnamese forces seized the Laotian towns of Attopeu and Saravane during the year, pushing what had been a 60 mi corridor to a width of 90 mi and opening the entire length of the Kong River system into Cambodia. A new logistical command, the 470th Transportation Group, was created to handle logistics in Cambodia and the new "Liberation Route" ran through Siem Prang and reached the Mekong at Stung Treng.

As foreseen by Secretary Laird, fallout from the incursion was quick in coming on the campuses of America's universities, as protests erupted against what was perceived as an expansion of the conflict into yet another country. On 4 May the unrest escalated to violence when Ohio National Guardsmen shot and killed four unarmed students (two of whom were not protesters) during the Kent State shootings. Two days later, at the University at Buffalo, police wounded four more demonstrators. On 15 May city and state police killed two and wounded twelve at Jackson State College (now Jackson State University) in Jackson, Mississippi. Earlier, on 8 May 100,000 protesters had gathered in Washington and another 150,000 in San Francisco on only ten days notice. Nationwide, 30 ROTC buildings went up in flames or were bombed while 26 schools witnessed violent clashes between students and police. National Guard units were mobilized on 21 campuses in 16 states. The student strike spread nationwide, involving more than four million students and 450 universities, colleges and high schools in mostly peaceful protests and walkouts.

Simultaneously, public opinion polls during the second week of May showed that 50 percent of the American public approved of President Nixon's actions. Fifty-eight percent blamed the students for what had occurred at Kent State. On both sides, emotions ran high. In one instance, in New York City on 8 May, pro-administration construction workers rioted and attacked demonstrating students. Such violence, however, was an aberration. Most demonstrations, both pro- and anti-war, were peaceful. On 20 May 100,000 construction workers, tradesmen, and office workers marched peacefully through New York City in support of the president's policies.

Reaction in the U.S. Congress to the incursion was also swift. Senators Frank F. Church (Democratic Party, Idaho) and John S. Cooper (Republican Party, Kentucky), proposed an amendment to the Foreign Military Sales Act that would have cut off funding not only for U.S. ground operations and advisors in Cambodia, but would also have ended U.S. air support for Cambodian forces. On 30 June the United States Senate passed the act with the amendment included. The bill was defeated in the House of Representatives after U.S. forces were withdrawn from Cambodia as scheduled. The newly amended act did, however, rescind the Southeast Asia Resolution (better known as the Gulf of Tonkin Resolution) under which Presidents Johnson and Nixon had conducted military operations for seven years without a declaration of war.

The Cooper–Church Amendment was resurrected during the winter and incorporated into the Supplementary Foreign Assistance Act of 1970. This time the measure made it through both houses of Congress and became law on 22 December. As a result, all U.S. ground troops and advisors were barred from participating in military actions in Laos or Cambodia, while the air war being conducted in both countries by the U.S. Air Force was ignored.

Conclusion
President Nixon proclaimed the incursion to be "the most successful military operation of the entire war." General Abrams was of like mind, believing that time had been bought for the pacification of the South Vietnamese countryside and that U.S. and ARVN forces had been made safe from any attack out of Cambodia during 1971 and 1972. A "decent interval" had been obtained for the final American withdrawal. ARVN General Tran Dinh Tho was more skeptical: "despite its spectacular results...it must be recognized that the Cambodian incursion proved, in the long run, to pose little more than a temporary disruption of North Vietnam's march toward domination of all of Laos, Cambodia, and South Vietnam."

John Shaw and other historians, military and civilian, have based the conclusions of their work on the incursion on the premise that the North Vietnamese logistical system in Cambodia had been so badly damaged that it was rendered ineffective. The next large-scale North Vietnamese offensive, the Nguyen Hue Offensive of 1972 (called the Easter Offensive in the West) would be launched out of southern North Vietnam and western Laos, not from Cambodia, was cited as proof positive that the Cambodian operations had succeeded. General Abrams claimed 11,000 enemy soldiers killed and 2,500 captured, but his death figure was disputed by CIA, who insisted that civilians death were figured into Abrams's total. The logistical haul discovered, removed, or destroyed in eastern Cambodia during the operations was indeed prodigious: 20,000 individual and 2,500 crew-served weapons; 7,000 to 8,000 tons of rice; 1,800 tons of ammunition (including 143,000 mortar shells, rockets, and recoilless rifle rounds); 29 tons of communications equipment; 431 vehicles; and 55 tons of medical supplies. MACV intelligence estimated that PAVN/VC forces in southern Vietnam required 1,222 tons of all supplies each month to keep up a normal pace of operations.

Due to the loss of its Cambodian supply system and continued aerial interdiction in Laos, MACV estimated that for every 2.5 tons of materiel sent south down the Ho Chi Minh trail, only one ton reached its destination. However, the true loss rate was probably only around ten percent. Due to lack of verifiable sources in the Socialist Republic of Vietnam, this figure is, at best, an estimate. The official Vietnamese history noted that "the enemy had established control over and successfully suppressed, to some extent at least, our nighttime supply operations. Enemy aircraft destroyed 4,000 trucks during the 1970-1971 dry season... Our supply effort, conducted during a single season of the year and using a single supply route, was unable to keep up with our requirements and our night supply operations encountered difficulties." Regardless, the PAVN's Group 559 successfully countered these efforts through camouflage tactics and the construction of thousands of kilometers of "bypass" roads to avoid choke points that frequently came under enemy attack. Per the same history, "in 1969 Group 559 shipped 20 thousand tons of supplies to the battlefields, in 1970 this total rose to 40 thousand tons and in 1971 it increased to 60 thousand tons... losses along the way in 1969, which were 13.5 percent, declined to 3.4 percent in 1970 and 2.7 percent in 1971." The U.S. Air Force's best estimate for the same time period was that one-third of the total amount was destroyed in transit.

South Vietnamese forces had performed well during the incursion but their leadership was uneven. General Trí proved a resourceful and inspiring commander, earning the sobriquet the "Patton of the Parrot's Beak" from the American media. General Abrams also praised the skill of General Nguyễn Viết Thanh, commander of IV Corps and planner of the Parrot's Beak operation. Unfortunately for the South Vietnamese, both officers were killed in helicopter crashes—Thanh on 2 May in Cambodia and Trí in February 1971. Other ARVN commanders, however, had not performed well. Even at this late date in the conflict, the appointment of ARVN general officers was prompted by political loyalty rather than professional competence. As a test of Vietnamization, the incursion was praised by American generals and politicians alike, but the Vietnamese had not really performed alone. The participation of U.S. ground and air forces had precluded any such claim. When called on to conduct solo offensive operations during the incursion into Laos (Operation Lam Son 719) in 1971, the ARVN's continued weaknesses would become all too apparent.

The Cambodian government was not informed of the incursion until it was already under way. The Cambodian leadership however welcomed the intervention against PAVN bases and the resulting weakening of PAVN military capabilities. The leadership had hoped for permanent US occupation of the PAVN sanctuaries because FANK and ARVN forces were unable to fill the vacuum in these territories following US withdrawal. It has been argued by some scholars that the incursion heated up the civil war and helped the insurgent Khmer Rouge gather recruits to their cause.

Unpublished government documents
Military Assistance Command, Vietnam, Command History 1967, Annex F. Saigon, 1968.

Published government documents

 * Foreign Relations Series
 * VIETNAM, JULY 1970–JANUARY 1972
 * VIETNAM, JANUARY 1969–JULY 1970