What event led to the United States entering World War II?

The entry of the United States into World War II was precipitated by the bombing of Pearl Harbor on December 7, 1941. This surprise military strike by the Japanese Imperial Navy aimed specifically at the U.S. naval base at Pearl Harbor in Hawaii resulted in significant loss of life and damage to the Pacific Fleet. The immediate effect of this attack was a strong wave of public outrage throughout the United States, leading to a swift decision by the U.S. government to declare war on Japan the following day, December 8, 1941. This event marked a pivotal moment, as it unified the nation and mobilized its military and industrial resources for the war effort.

The other events listed, while historically significant, did not directly lead to the entrance of the United States into World War II. The sinking of the Lusitania occurred during World War I, the invasion of Poland sparked the war in Europe in 1939 but did not involve the U.S. until later, and the attack on Fort Sumter was a key event in the American Civil War, not World War II. Understanding the context of Pearl Harbor is essential in grasping why it was the decisive moment for the U.S. in the global conflict.