Nagato-class battleship

The Nagato-class battleships (長門型戦艦) were a pair of dreadnought battleships built for the Imperial Japanese Navy (IJN) during World War I, although they were not completed until after the end of the war. JAPANESE BATTLESHIP Nagato, the lead ship of the class, frequently served as a flagship. Both ships carried supplies for the survivors of the Great Kantō earthquake in 1923. They were modernized in 1933–36 with improvements to their armor and machinery and a rebuilt superstructure in the pagoda mast style. Nagato and her sister ship JAPANESE BATTLESHIP Mutsu briefly participated in the Second Sino-Japanese War in 1937 and Nagato was the flagship of Admiral Isoroku Yamamoto during the attack on Pearl Harbor on 7 December 1941 that began the Pacific War.

The sisters participated in the Battle of Midway in June 1942, although they did not see any combat. Mutsu saw more active service than her sister because she was not a flagship and participated in the Battle of the Eastern Solomons in August before returning to Japan in early 1943. One of Mutsu's aft magazines detonated in June, killing 1,121 crew and visitors and destroying the ship. The IJN conducted a perfunctory investigation into the cause of her loss and concluded that it was the work of a disgruntled crewmember. They dispersed the survivors in an attempt to conceal the sinking to keep up morale in Japan. Much of the wreck was salvaged after the war and many artifacts and relics are on display in Japan.

Nagato spent most of the first two years of the war training in home waters. She was transferred to Truk in mid-1943, but did not see any combat until the Battle of the Philippine Sea in mid-1944 when she was attacked by American aircraft. Nagato did not fire her main armament against enemy vessels until the Battle of Leyte Gulf in October 1944. She was lightly damaged during the battle and returned to Japan the following month for repairs. The IJN was running out of fuel by this time and decided not to fully repair her. Nagato was converted into a floating anti-aircraft platform and assigned to coastal defense duties. After the war, the ship was a target for U.S. nuclear weapon tests during Operation Crossroads in mid-1946. She survived the first test with little damage, but was sunk by the second test.

Background
The IJN had considered a battle fleet of eight modern battleships and eight modern armored cruisers necessary for the defense of Japan and the government ratified that policy in 1907. This was the genesis of the Eight-Eight Fleet Program, the development of a cohesive battle line of sixteen capital ships less than eight years old. Advances in naval technology like the British battleship HMS Dreadnought (1906), and the battlecruiser HMS Invincible (1907) forced the IJN to reevaluate the ships that it counted as modern several times. By 1910 the IJN considered that none of its current ships were modern and restarted the program in 1911 with orders for the dreadnoughts and the s. By 1915, the IJN was halfway to its goal and wanted to order four more dreadnoughts, but the Diet rejected its plan, although it authorized one dreadnought, later named JAPANESE BATTLESHIP Nagato, and two battlecruisers in the 1916 budget. Later that year American President Woodrow Wilson announced plans for ten additional battleships and six battlecruisers and the Diet authorized three more dreadnoughts in response the following year, one of which would later be named JAPANESE BATTLESHIP Mutsu.

Design
Allocated project number A-102, the Nagato class was designed before Commander Yuzuru Hiraga was reassigned to the Navy Technical Department (NTD) responsible for ship design. In contrast to earlier designs, the Nagato class used the American "all or nothing" armor scheme that maximized the armor thickness protecting the core of the ship by eliminating armor elsewhere. The ships also used two armored decks of medium thickness rather than the single thicker deck used formerly. The ships also used a new type of underwater protection system that successfully resisted penetration by 200 kg torpedo warheads in full-sized trials. It consisted of a deep water-tight compartment adjacent to the hull, backed by a thick torpedo bulkhead that connected to the side and deck armor plates, with a deep fuel oil tank behind it.

Although the United States Navy planned to arm its s with 16 in guns before the Nagato class was designed, Nagato's 41 cm guns made her the first dreadnought launched to be armed with guns larger than 15 in.

On 12 June 1917, well before Mutsu was laid down, Hiraga proposed a revised design for the ship that reflected the lessons from the Battle of Jutland a year previously as well as incorporating advances in boiler technology. Given project number A-125, his design added an extra twin main-gun turret, using space and weight made available by the reduction of the number of boilers from 21 to 12 while the power remained the same. He reduced the secondary armament from 20 guns to 16, although they were raised in height to improve their ability to fire during heavy weather and to improve their arc of fire. To increase the ship's protection he proposed to angle the belt armor outwards to improve its resistance to horizontal fire and to increase the thickness of the lower deck armor and the torpedo bulkhead. Hiraga also planned to add anti-torpedo bulges to improve its underwater protection. He estimated that his ship would displace as much as Nagato, although it would cost about a million yen more. Hiraga's changes would have considerably delayed Mutsu's completion and were rejected by the Navy Ministry.

Description
The ships had a length of 201.17 m between perpendiculars and 215.8 m overall. They had a beam of 29.02 m and a draft of 9.08 m. The Nagato-class ships displaced 32720 t at standard load and 39116 t at full load. Their crew consisted of 1,333 officers and enlisted men as built and 1,368 in 1935. During World War II, the crew totalled around 1,734 men in 1944.

The Nagato class was equipped with a unique heptapodal (seven-legged) mast designed to maximize rigidity for range-finding purposes and survivability under shellfire. It consisted of a thick vertical leg in the center surrounded by six outer legs. The central leg was large enough to accommodate an electric elevator running between the foretop and main deck. In November 1944, the tops of Nagato's mainmast and funnel were removed to improve the arcs of fire for her anti-aircraft guns.

In 1927, Mutsu's bow was remodeled to reduce the amount of spray produced when steaming into a head sea. This increased her overall length by 1.59 m to 217.39 m. This proved successful and her sister's bow was rebuilt in 1930. During their 1934–36 reconstruction, the ships' sterns were lengthened by 7.55 m to improve their speed and their forward superstructures were rebuilt into a pagoda mast. They were given torpedo bulges to improve their underwater protection and to compensate for the weight of the additional armor and equipment. These changes increased their overall length to 224.94 m, their beam to 34.6 m and their draft to 9.49 m. Their displacement increased over 7000 t to 46690 t at deep load.

Propulsion
The ships were equipped with four Gihon geared steam turbines, each of which drove one 4.191 m propeller. The turbines were designed to produce a total of 80000 shp, using steam provided by 21 Kampon water-tube boilers; 15 of these were oil-fired while the remaining half-dozen consumed a mixture of coal and oil. The boilers had a working pressure of 286 psi.

The ships had a stowage capacity of 1600 LT of coal and 3400 LT of fuel oil, giving them a range of 5500 nmi at a speed of 16 kn. Nagato exceeded her designed speed of 26.5 knots during her sea trials, reaching 26.7 kn at 85500 shp and Mutsu reached the same speed with 87500 shp. The US Navy did not learn until about 1937 just how fast they were; previously it had believed that the ships were only capable of 23 kn.

During refits in 1923–25, the fore funnel was rebuilt in a serpentine shape in an unsuccessful effort to prevent smoke interference with the bridge and fire-control systems. That funnel was eliminated during the ships' 1930s reconstructions when their boilers were replaced by ten oil-fired Kampon boilers, which had a working pressure of 313 psi and temperature of 300 °C. In addition her turbines were replaced by lighter, more modern, units and their propellers were replaced by larger 4.3 m ones. When Mutsu conducted her post-reconstruction trials, she reached a speed of 24.98 kn with 82300 shp. Additional fuel oil was stored in the bottoms of the newly added torpedo bulges, which increased their capacity to 5560 LT and thus their range to 8560 nmi at 16 knots.

Armament
The Nagato-class ships were armed with eight 45-caliber 41-centimeter guns, mounted in two pairs of twin-gun, superfiring turrets fore and aft. Numbered one through four from front to rear, the hydraulically powered turrets gave the guns an elevation range of −2 to +35 degrees. The rate of fire for the guns was around two rounds per minute. Their turrets were replaced in the mid-1930s using the turrets stored from the unfinished ships. While in storage the turrets were modified to increase their range of elevation to −3 to +43 degrees, which increased the gun's maximum range from 30200 to 37900 m.

By World War II, the guns used Type 91 armor-piercing, capped shells. Each of these shells weighed 1020 kg and had a muzzle velocity of 780 m/s. Also available was a 936 kg high-explosive shell that had a muzzle velocity of 805 m/s. A special Type 3 Sankaidan incendiary shrapnel shell was developed in the 1930s for anti-aircraft use.

The ships' secondary armament of twenty 50-caliber 14-centimeter guns was mounted in casemates on the upper sides of the hull and in the superstructure. The latter guns had a maximum elevation of +20 degrees and the former could elevate to +25 degrees which gave them ranges of 15800 m and 17000 m respectively. Each gun could fire a 38 kg high-explosive projectile at a rate up to 10 rounds per minute. Anti-aircraft defense was provided by four 40-caliber 3rd Year Type three-inch AA guns in single mounts. The 3 in high-angle guns had a maximum elevation of +75 degrees, and had a rate of fire of 13 to 20 rounds per minute. They fired a 6 kg projectile with a muzzle velocity of 680 m/s to a maximum height of 7500 m. The ship was also fitted with eight 53.3 cm torpedo tubes, four on each broadside, two above water and two submerged. They used the 6th Year Type torpedo which had a 203 kg warhead of Shimose powder. It had three settings for range and speed: 15500 m at 26 kn, 10000 m at 32 kn, or 7000 m at 37 kn.

Around 1926, the four above-water torpedo tubes were removed and the ships received three additional 76 mm AA guns that were situated around the base of the foremast. They were replaced by eight 40-caliber 12.7-centimeter Type 89 dual-purpose guns in 1932, fitted on both sides of the fore and aft superstructures in four twin-gun mounts. When firing at surface targets, the guns had a range of 14700 m; they had a maximum ceiling of 9440 m at their maximum elevation of +90 degrees. Their maximum rate of fire was 14 rounds a minute, but their sustained rate of fire was around eight rounds per minute. Two twin-gun mounts for license-built Vickers two-pounder light AA guns were also added to the ship that same year. These guns had a maximum elevation of +80 degrees which gave them a ceiling of 4000 m. They had a maximum rate of fire of 200 rounds per minute.

When the ships were reconstructed in 1934–36, the remaining torpedo tubes and the two forward 14 cm guns were removed from the hull. All of the remaining 14 cm guns had their elevation increased to +35 degrees which increased their range to 20000 m. An unknown number of license-built 13.2 mm Hotchkiss machine guns in twin mounts were added. The maximum range of these guns was 6500 m, but the effective range against aircraft was 700 - 1500 m. The cyclic rate was adjustable between 425 and 475 rounds per minute, but the need to change 30-round magazines reduced the effective rate to 250 rounds per minute.

The unsatisfactory two-pounders were replaced in 1939 by twenty license-built Hotchkiss 25 mm Type 96 light AA guns in a mixture of twin-gun and single mounts. This was the standard Japanese light AA gun during World War II, but it suffered from severe design shortcomings that rendered it a largely ineffective weapon. According to historian Mark Stille, the twin and triple mounts "lacked sufficient speed in train or elevation; the gun sights were unable to handle fast targets; the gun exhibited excessive vibration; the magazine was too small, and, finally, the gun produced excessive muzzle blast". These 25 mm guns had an effective range of 1500 - 3000 m, and an effective ceiling of 5500 m at an elevation of 85 degrees. The maximum effective rate of fire was only between 110 and 120 rounds per minute because of the frequent need to change the fifteen-round magazines. As far as is known, no additional AA guns were installed aboard Mutsu before her loss. Additional 25 mm guns were installed aboard Nagato during the war; on 10 July 1944, she was reported to have 98 guns on board. An additional 30 guns were added during a refit in Yokosuka in November. Two more twin 127 mm gun mounts are added at the same time abreast the funnel and her 14 cm guns were removed as she was now a floating anti-aircraft battery.

Armor
The Nagato-class ships' waterline armor belt was made from Vickers cemented armor and protected 137.14 m of the hull between Barbette No. 1 and Barbette No. 4. The lower strake was 305 mm thick, 2.7 m high, and tapered to a thickness of 100 mm at its bottom edge; above it was a strake of 229 mm armor that was 1.7 m high. Approximately 1.77 m of the armor belt was below the waterline. The turrets were protected with an armor thickness of 305 mm on the face, 230 - 190 mm on the sides, and 152 - 127 mm on the roof. The barbettes of the turrets were protected by armor 305 mm thick, while the casemates of the 140 mm guns were protected by 25 mm armor plates. The sides of the conning tower were 369 mm thick.

The main deck armor consisted of three layers of high-tensile steel (HTS) 69 mm thick that connected to the top of the upper strake of side armor. The flat portion of the lower deck had one layer of Ducol steel 25 mm thick with two layers of HTS of equal thickness above it. About 3 m from the side of the hull, this deck, now composed of three layers of HTS, totalling 75 mm in thickness, sloped downwards where it met a short horizontal armored (three layers of HTS with a total thickness of 66 mm) deck that connected to the main armored belt and the torpedo bulkhead. This was also made up of three 25 mm layers of HTS and curved up and outwards to meet the short horizontal armored deck. It enclosed a water-tight compartment that was 3.05 m from the side of the ship. It was backed by fuel oil tanks 3.13 m deep. The outermost void was designed to allow the explosive force of a torpedo's warhead to dissipate as much as possible while the oil tank was supposed to stop any fragments from penetrating the innermost bulkhead protecting the ship's vital areas.

The new 41 cm turrets installed during their reconstruction were more heavily armored than the original ones. Face armor was increased to 460 mm, the sides to 280 mm, and the roof to 250 - 230 mm. The armor over the machinery and magazines was increased by 38 mm on the upper deck and 25 mm on the upper armored deck. The torpedo bulges added at the same time were 13.5 m high and 2.84 m deep. They were divided into four compartments, the lower two of which were filled with oil and the others remained empty. These additions increased the weight of each ship's armor to 13032 t, 32.6 percent of their displacement. In early 1941, as a preparation for war, the uppermost compartment of the bulges was filled with sealed steel crushing tubes and the barbette armor of both ships was reinforced with 100 mm armor plates above the main deck and 215 mm plates below it.

Fire control and sensors
The Nagato-class ships were fitted with a 10 m rangefinder in the forward superstructure. and anti-aircraft rangefinders were in the early 1920s. The rangefinders in No. 2 and 3 Turrets were replaced by 10-meter units in 1932–33.

They were initially fitted with a Type 13 fire-control system derived from Vickers equipment received during World War I, but this was replaced by an improved Type 14 system around 1925. It controlled the main and secondary guns; no provision was made for anti-aircraft fire until the Type 31 fire-control director was introduced in 1932. A modified Type 14 fire-control system was tested aboard Nagato in 1935 and later approved for service as the Type 34. A new anti-aircraft director, the Type 94 used to control the 127 mm AA guns, was introduced in 1937, although when the ships received theirs is unknown. The 25 mm AA guns were controlled by a Type 95 director that was also introduced in 1937.

As far as is known, no radars were installed aboard Mutsu before her loss. While in drydock in May 1943, a Type 21 air-search radar was installed aboard Nagato on the roof of the 10-meter rangefinder at the top of the pagoda mast. On 27 June 1944, two Type 22 surface-search radars were installed on the pagoda mast and two Type 13 early warning radars were fitted on her mainmast.

Aircraft
Nagato was briefly fitted with a 18 m aircraft flying-off platform on Turret No. 2 in August 1925. Yokosuka Ro-go Ko-gata and Heinkel HD 25 floatplanes were tested from it before it was removed early the following year. An additional boom was added to the mainmasts of both ships in 1926 to handle the Yokosuka E1Y then assigned to them. A Hansa-Brandenburg W.33 floatplane was tested aboard Nagato that same year. A catapult was fitted between the mainmast and Turret No. 3 in mid-1933, a collapsible crane was installed in a portside sponson, and the ships were equipped to operate two or three floatplanes, although no hangar was provided. They now operated Nakajima E4N2 biplanes until they were replaced by Nakajima E8N2 biplanes in 1938. A more powerful catapult was installed in November 1938 to handle heavier aircraft like the one Kawanishi E7K that was added in 1939–40. Mitsubishi F1M biplanes replaced the E8Ns on 11 February 1943.

Construction and service
While Mutsu was still fitting out, the American government decided to call a conference in Washington, D.C. later in 1921 to forestall the massively expensive naval arms race between the United States, the United Kingdom and the Empire of Japan that was developing. The Washington Naval Conference convened on 12 November and the Americans proposed to scrap virtually every capital ship under construction or fitting out among the participating nations. Mutsu was specifically listed among those to be scrapped even though she had been commissioned a few weeks earlier. This was unacceptable to the Japanese delegation and they agreed to a compromise that allowed them to keep Mutsu in exchange for scrapping the obsolete semi-dreadnought JAPANESE BATTLESHIP Settsu and a similar arrangement for several American Colorado-class dreadnoughts that were fitting out.

Upon commissioning, the sister ships were assigned to the 1st Battleship Division, although Nagato became the flagship of Rear Admiral Sōjirō Tochinai, a role she often fulfilled during her career. The ships hosted the Prince of Wales Edward and his aide-de-camp Lieutenant Louis Mountbatten on 12 April during the prince's visit to Japan. After the 1923 Great Kantō earthquake, both ships loaded supplies from Kyushu for the victims on 4 September. They sank the hulk of the obsolete battleship JAPANESE BATTLESHIP Satsuma on 7 September 1924 during gunnery practice in Tokyo Bay in accordance with the Washington Naval Treaty. Nagato became flagship of the Combined Fleet on 1 December 1925, flying the flag of Admiral Keisuke Okada. The sisters were placed in reserve several times during the 1920s while they were being modernized. Mutsu served as flagship of Emperor Hirohito during the 1927 naval maneuvers and fleet review.

Mutsu again served as the Emperor's flagship during the annual maneuvers and fleet review in 1933. The ships were reconstructed from late 1933 to mid-1936. In August 1937, the sisters transported 3,749 men of the 11th Infantry Division to Shanghai during the Second Sino-Japanese War. Their floatplanes bombed targets in Shanghai on 24 August before they returned to Sasebo the following day. Nagato became a training ship on 1 December 1937 until she again became the flagship of the Combined Fleet on 15 December 1938. The ship participated in an Imperial Fleet Review on 11 October 1940. The sisters were refitted in 1941 in preparation for war, which included the fitting of external degaussing coils and additional armor for her barbettes.

World War II
Admiral Isoroku Yamamoto issued the code phrase "Niitaka yama nobore" (Climb Mount Niitaka) on 2 December 1941 from Nagato at anchor at Hashirajima to signal the 1st Air Fleet (Kido Butai) to proceed with its attack on Pearl Harbor. When Japan began the Pacific War on 8 December, the sisters sortied for the Bonin Islands, along with the four ships of Battleship Division 2 and the light carrier JAPANESE AIRCRAFT CARRIER Hōshō as distant support for the fleet attacking Pearl Harbor, and returned six days later. Yamamoto transferred his flag to the new battleship JAPANESE BATTLESHIP Yamato on 12 February 1942.

In June 1942 Mutsu and Nagato were assigned to the Main Body of the 1st Fleet during the Battle of Midway, together with Yamato, Hosho, escorted by the light cruiser Sendai and nine destroyers. Following the loss of all four carriers on 4 June, Yamamoto attempted to lure the American forces west to within range of the Japanese air groups at Wake Island, and into a night engagement with his surface forces, but the American forces withdrew and Mutsu saw no action. After rendezvousing with the remnants of the Striking Force on 6 June, over half of the survivors from the sunken aircraft carriers of the 1st Air Fleet were transferred to Mutsu and Nagato. They arrived at Hashirajima on 14 June.

On 14 July both ships were reassigned to Battleship Division 2 and Nagato became the flagship of the 1st Fleet. She remained in Japanese waters training until August 1943. Mutsu was transferred to the Advance Force of the 2nd Fleet on 9 August and departed Yokosuka two days later to support operations during the Guadalcanal Campaign. They arrived at Truk on 17 August. On 20 August, while sailing from Truk to rendezvous with the main body of Vice Admiral Chūichi Nagumo's 3rd Fleet, Mutsu, the heavy cruiser JAPANESE CRUISER Atago, and escorting destroyers unsuccessfully attempted to locate the escort carrier USS Long Island (CVE-1) in response to a flying boat detecting the American ship.

During the Battle of the Eastern Solomons on 27 August, Mutsu, assigned to the Support Force, fired four shells at enemy reconnaissance aircraft during what was her first and only action of the war. On 7 January 1943, Mutsu returned to Japan together with the carrier JAPANESE AIRCRAFT CARRIER Zuikaku, the heavy cruiser JAPANESE CRUISER Suzuya and four destroyers. The ship prepared to sortie on 13 April to reinforce the Japanese garrisons in the Aleutian Islands in response to the Battle of the Komandorski Islands, but the operation was cancelled the next day and Mutsu resumed training.

Mutsu's loss
On 8 June 1943, Mutsu was moored at Hashirajima when the magazine of her No. 3 turret exploded at 12:13, cutting the ship in half. The forward section capsized almost immediately, but the rear section remained afloat until the early morning of the next day. Nearby ships were able to rescue 353 survivors from the 1,474 crew members and visitors aboard Mutsu, meaning that 1,121 men were killed in the explosion. To avert the potential damage to morale from the loss of a battleship, Mutsu's loss was declared a state secret. To further prevent rumours from spreading, many survivors were re-assigned to various garrisons in the Pacific Ocean.

The IJN convened a commission three days after the sinking to investigate the loss. It issued its preliminary conclusions on 25 June, well before the investigation of the wreck was completed, and decided that the explosion was the result of a disgruntled seaman. The commission failed to consider the possibility of fire which historian Mike Williams considers to be a possible cause as a number of observers noted smoke coming from the vicinity of No. 3 turret. The truth, however, will never be known.

Nagato
Nagato transferred to Truk in the Caroline Islands in August 1943. Together with the bulk of the 1st Fleet, she sortied in September and October in unsuccessful searches for American carriers. On 1 February 1944, Nagato departed Truk to avoid an American air raid, and arrived at Palau three days later. The ship arrived on 21 February at Lingga Island, near Singapore, and she became the flagship of Vice Admiral Matome Ugaki, commander of Battleship Division 1, on 25 February until he transferred his flag to Yamato on 5 May. Nagato remained at Lingga until 11 May when she was transferred to Tawitawi on 12 May and assigned to the 1st Mobile Fleet, under the command of Vice Admiral Jisaburō Ozawa.

Battleship Division 1 rendezvoused with Ozawa's main force on 16 June near the Mariana Islands and Nagato escorted three aircraft carriers during the Battle of the Philippine Sea. She was only lightly engaged during the battle and was not damaged nor suffered any casualties. After the battle, the ship returned to Japan where she was refitted with additional radars and light AA guns. Nagato loaded a regiment of the 28th Infantry Division and delivered them at Okinawa on 11 July before continuing on to Lingga.

In October 1944, the ship took part in "Operation Sho-1", an attack on the Allied landings on Leyte. Assigned to Vice Admiral Takeo Kurita's 1st Diversion Force (also known as the Center Force), Nagato was attacked by multiple waves of American dive bombers and fighters during the Battle of the Sibuyan Sea on 24 October en route to Leyte Gulf. She was struck twice by bombs that killed 52 crewmen, but was not seriously damaged.

The next morning, the 1st Diversion Force attacked the American forces supporting the invasion in the Battle off Samar. Nagato engaged the escort carriers and destroyers of Task Group 77.4.3, codenamed "Taffy 3". She opened fire on three escort carriers, the first time she had ever fired her guns at an enemy ship, but missed. One of the defending destroyers fired a spread of torpedoes that missed their intended target and headed for Yamato and Nagato which were on a parallel course. The two battleships were forced to turn away north to avoid the torpedoes, and were forced 10 miles (16 km) away from the engagement before the torpedoes ran out of fuel. Turning back, Nagato engaged the American ships, claiming to have damaged one cruiser. Later in the day, Nagato was hit in the bow by two bombs, but the damage was not severe.

The ship returned to Japan for repairs in mid-November. Lack of fuel and materials meant that she could not be brought back into service and she was turned into a floating anti-aircraft battery. Her funnel and mainmast were removed to improve the arcs of fire of her AA guns, which were increased by two Type 89 mounts and nine triple 25 mm gun mounts. Her forward secondary guns were removed in compensation. A coal-burning donkey boiler was installed pierside for heating and cooking purposes and a converted submarine chaser was moored alongside to provide steam and electricity; her anti-aircraft guns lacked full power and were only partially operational. On 20 April Nagato was reduced to reserve. In June 1945, all of her secondary guns and about half of her anti-aircraft armament was moved ashore, together with her rangefinders and searchlights. Her crew was therefore reduced to less than 1,000 officers and enlisted men. On 18 July 1945, the heavily camouflaged ship was attacked by carrier-based fighter bombers and torpedo bombers. Nagato was hit by two bombs and a rocket that killed 35 men and damaged four 25 mm guns. A few days before the formal surrender, American sailors took control of the ship on 30 August.

After the war
Nagato was selected to participate as a target ship in Operation Crossroads, a series of U.S. nuclear weapon tests held at Bikini Atoll in mid-1946. In mid-March, Nagato departed Yokosuka for Eniwetok, but her hull had not been repaired from the underwater damage sustained during the attack on 18 July and she leaked enough that her pumps could not keep up. The ship had a list of seven degrees to port by the time tugboats from Eniwetok arrived. She reached the atoll on 4 April and Bikini in May.

Operation Crossroads began with the first blast (Test Able), an air burst on 1 July 1946; Nagato was not close to ground zero and was only lightly damaged. For Test Baker, an underwater explosion, the ship was positioned closer to ground zero. Nagato rode out the tsunami of water from the explosion with little apparent damage; she had a slight starboard list of two degrees after the tsunami dissipated. A more thorough assessment could not be made because she was dangerously radioactive. Her list gradually increased over the next five days and she capsized during the night of 29/30 July. The Times has named Nagato as one of the top ten wreck diving sites in the world.

In 1970, salvage operations began on Mutsu's wreck that lasted until 1978 and recovered about 75% of the ship. The salvagers recovered bodies of 849 crewmen killed during the explosion. In 1995, the Mutsu Memorial Museum declared that no further salvage operations were planned. The only significant portion of the ship that remains is a 35 m long section running from the bridge structure forward to the vicinity of No. 1 turret. The highest portion of the ship is 12 m below the surface. Many, but not all, artifacts are displayed at the Mutsu Memorial Museum in Tōwa-Cho. Since 1963, a memorial service has been held here every year on 8 June in honor of the crew.