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

The attack on Pearl Harbor is the pivotal event that led to the United States' formal entry into World War II. On December 7, 1941, the Japanese conducted a surprise military attack on the U.S. naval base at Pearl Harbor in Hawaii. This devastating assault resulted in significant loss of life and damage to the Pacific Fleet, overwhelming American defenses. The day after the attack, the United States declared war on Japan, marking its entry into the global conflict.

In the context of the other events listed, the sinking of the Lusitania occurred during World War I, which had no direct relevance to the U.S. entry into World War II. The invasion of Poland in 1939 marked the beginning of World War II in Europe, but it did not provoke an immediate American response. The Treaty of Versailles, which ended World War I, is also unrelated to the events that led to U.S. involvement in the second global conflict. Thus, the attack on Pearl Harbor distinctly stands as the catalyst for the U.S. entering World War II.