Which 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. This surprise military strike by the Japanese Navy on December 7, 1941, targeted the naval base at Pearl Harbor in Hawaii, leading to significant loss of life and damage to the United States Pacific Fleet. The attack galvanized public opinion and unified the nation, eliminating any remaining isolationist sentiment and leading to a formal declaration of war against Japan the following day. This action marked the United States' active involvement in World War II, shifting the dynamics of the conflict on a global scale.

The other events listed, such as the bombing of Hiroshima, the invasion of Poland, and the signing of the Treaty of Versailles, while historically significant, did not directly trigger U.S. entry into the war. The bombing of Hiroshima occurred much later in the war as part of the effort to bring Japan to surrender. The invasion of Poland was a crucial event that initiated the war in Europe, but the U.S. did not enter the conflict until after the attack on Pearl Harbor. The Treaty of Versailles was a peace treaty that officially ended World War I, which played a role in the geopolitical landscape but was not a direct cause for U.S. involvement in World War