Which event is often cited as a catalyst for U.S. entry into World War II?

The attack on Pearl Harbor is regarded as a pivotal catalyst for the United States' entry into World War II due to its immediate and profound impact on American public opinion and government policy. On December 7, 1941, the Japanese naval forces launched a surprise military strike against the U.S. naval base at Pearl Harbor in Hawaii, resulting in significant loss of life and destruction of naval assets. This attack galvanized the American public, which had been largely isolationist and hesitant to join the conflict.

In response, President Franklin D. Roosevelt and Congress moved quickly to declare war on Japan on December 8, 1941, marking a decisive shift in U.S. foreign policy. The surprise nature of the attack, along with the scale of its destruction, created a sense of urgency and unity among the American populace, making it clear that the nation could no longer remain disengaged from the global conflict.

The other events mentioned—such as the sinking of the Lusitania, the bombing of Hiroshima, and the signing of the Treaty of Versailles—are significant in their own right but do not have the same direct connection to the U.S. decision to enter World War II. The Lusitania incident occurred during World War I and influenced American sentiment at the