What event prompted the United States to enter World War II?

The event that prompted the United States to enter World War II was the attack on Pearl Harbor, which took place on December 7, 1941. This surprise military strike by the Imperial Japanese Navy resulted in significant loss of life and damage to the U.S. Pacific Fleet, serving as a crucial catalyst for American involvement in the conflict. In the wake of the attack, public sentiment quickly shifted to overwhelming support for entering the war, leading President Franklin D. Roosevelt to deliver a speech to Congress the following day, famously calling it "a date which will live in infamy."

The significance of this event lies not just in the immediate military and strategic implications but also in the way it unified the nation in a common cause, marking a decisive turning point in U.S. foreign policy from isolationism to active engagement in World War II. In contrast, the other options, such as the signing of the Treaty of Versailles or the sinking of the Lusitania, relate to different historical contexts and conflicts. The D-Day invasion occurred later in the war, representing a major Allied offensive rather than the U.S. entry into the war. Thus, the attack on Pearl Harbor is firmly recognized as the event that triggered the U.S. involvement in World War II