A series of events led to the attack on Pearl Harbor. War between the Empire of Japan and the United States was a possibility each nation's military forces had planned for after World War I. The expansion of American territories in the Pacific had been a threat to Japan since the 1890s, but real tensions did not begin until the Japanese invasion of Manchuria in 1931.

The Tripartite Pact, also known as the Berlin Pact, was an agreement between Germany, Italy, and Japan signed in Berlin on 27 September 1940 by, respectively, Joachim von Ribbentrop, Galeazzo Ciano, and Saburō Kurusu and in the presence of Adolf Hitler. It was a defensive military alliance that was eventually joined by Hungary, Romania, Slovakia, Bulgaria, and Yugoslavia. Yugoslavia's accession provoked a coup d'état in Belgrade two days later. Germany, Italy, and Hungary responded by invading Yugoslavia. The resulting Italo-German client state, known as the Independent State of Croatia, joined the pact on 15 June 1941.