How did the Tripartite Pact affect Germany’s decision to declare war on the United States?

The Tripartite Pact, signed in September 1940, created a defense alliance between Germany, Italy, and Japan, with the primary intent of deterring the United States from entering World War II. The pact obligated the signatories to assist one another if any member was attacked by a power not then involved in the European war or the Sino-Japanese conflict—a provision specifically aimed at the United States.12

Following the Japanese attack on Pearl Harbor on December 7, 1941, and the subsequent U.S. declaration of war on Japan, Germany and Italy declared war on the United States on December 11, 1941.13

It is important to note that the pact did not technically mandate that Germany declare war, as the agreement only obligated members to defend one another if they were attacked, not if they were the aggressors. Despite this, and the fact that German armies were already stalled on the Eastern Front, Adolf Hitler chose to honor the alliance and declare war on the United States. Ultimately, however, the pact was largely ineffective, as Germany and Japan pursued divergent interests and agendas throughout the conflict.412