First engagement of neutral United States in World War II before the attack on Pearl Harbor

Determining when the first engagement of neutral United States in World War II before the attack on Pearl Harbor occurred depends on the scholar and if such actions led to formal entry of the United States into the conflict.

Attacks on Americans
The attack on the United States gunboat USS Panay (PR-5) on 12 December 1937 by Japanese forces in China (usually referred to as the Panay incident) could be considered as the first hostile American action during World War II. Although the war was not officially declared in Europe until Germany invaded Poland on 1 September 1939, Japan had been involved in military actions against China since 1931. On January 26, 1938, in what was known as the Allison incident, John M. Allison, at the time consul at the American embassy in Nanking, was struck in the face by a Japanese soldier. Even though the Japanese apologized formally on January 30 (after the Americans demanded they do so), this incident, together with the looting of American property in Nanking that took place at the same time, further strained relations between Japan and the United States, which had already been damaged by the Panay incident less than two months earlier.

The first American military death in the European Theatre occurred on 21 April 1940, during the German invasion of Norway. Military attaché Captain Robert M. Losey was killed during a German bombardment of Dombås while assisting with the evacuation of U.S. embassy personnel and others to Sweden.

Either the casualties inflicted on USS Kearny (DD-432) by German U-boat U-568 on 17 October 1941, or the sinking of the USS Reuben James (DD-245) by U-552 on 31 October 1941 might be considered the first American naval losses of World War II. The United States was neither officially involved in the war at the time nor did the incidents cause them to declare war.

Attacks by the U.S. military
The first American hostile action against Axis forces was on 10 April 1941, when the destroyer USS Niblack (DD-424) attacked a German U-boat that had just sunk a Dutch freighter. USS Niblack was picking up survivors of the freighter when it detected a U-boat preparing to attack. The Niblack attacked with depth charges and drove off the U-boat. There were no casualties on board USS Niblack or the U-boat. By coincidence, USS Niblack was later in the same convoy as USS Reuben James when that ship was sunk, and picked up survivors from USS Reuben James.

The first American hostile action against Axis forces that resulted in physical destruction was on 14 September 1941, when USCGC Northland (WPG-49) destroyed a German weather station in northeast Greenland. The action was based on an agreement with Denmark Ambassador to the United States Henrik Kauffmann in April 1941 to patrol the Danish island.

The first American-caused casualties occurred on 7 December 1941 when the USS Ward attacked and sank a Japanese midget submarine near the entrance to Pearl Harbor prior to the commencement of the Japanese air attack upon Hawaii later that day. As a result of the attack on Hawaii, America declared war on Japan on 8 December 1941. Germany and Italy declared war on the United States three days later.

The first planned offensive action by the United States in World War II came in January 1942 when the aircraft carrier USS Enterprise (CV-6) attacked Japanese bases in the Marshall Islands.