Italian declaration of war on the United States in the context of "Galeazzo Ciano"

Japan's attack on Pearl Harbor took place on December 7, 1941. The United States military suffered 19 ships damaged or sunk, and 2,403 people were killed. Its most significant consequence was the entrance of the United States into World War II. The US had previously been officially neutral and considered an isolationist country with its Neutrality Act but subsequently after the attack declared war on Japan the next day and entered the Pacific War. Then on December 11, 1941, four days after the Japanese attack, after the Italian declaration of war on the United States and the German declaration of war against the United States, which Hitler had orchestrated, the US was then at war with Germany and Italy. The US then became involved in the Battle of the Atlantic and the European theatre of war as well as with Japan.

Following the attack, the US interned 120,000 Japanese Americans, 11,000 German Americans, and 3,000 Italian Americans.

On 11 December 1941, four days after the Japanese attack on Pearl Harbor and three days after the United States declaration of war against Imperial Japan, Nazi Germany declared war against the United States, in response to what was claimed to be a "series of provocations" by the United States government when the U.S. was still officially neutral during World War II. The decision to declare war was made by Adolf Hitler, following two days of consultation. It has been referred to as Hitler's "most puzzling" decision of World War II. Publicly, the formal declaration was made to American Chargé d'affaires Leland B. Morris by German Foreign Minister Joachim von Ribbentrop in the latter's office. Benito Mussolini also announced Italy's declaration of war against the United States on 11 December. Later that day, the U.S. declared war on Germany, with Germany's action having eliminated any remaining meaningful domestic isolationist opposition to the U.S. joining the European war.