On 11 December 1941, four days after the Japanese attack on Pearl Harbor and the United States declaration of war against the Japanese Empire, 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, apparently offhand, almost without 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. 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.

The attack on Pearl Harbor was a surprise military strike by the Imperial Japanese Navy Air Service on the American naval base at Pearl Harbor in Honolulu, Hawaii, in the United States, just before 8:00 a.m. on Sunday, December 7, 1941. At the time, the United States was a neutral country in the World War II conflict. The attack on Hawaii and other U.S. territories led the United States to formally enter World War II on the side of the Allies the day following the attack, on December 8, 1941. The Japanese military leadership referred to the attack as the Hawaii Operation and Operation AI, and as Operation Z during its planning.