Why did the United States enter World War II?

The entry of the United States into World War II was primarily catalyzed by the attack on Pearl Harbor on December 7, 1941. This surprise military strike by the Japanese Navy aimed to incapacitate the U.S. Pacific Fleet and was a decisive moment in American history. The attack resulted in significant loss of life and damage to the fleet, provoking a strong public and governmental response.

Following the attack, President Franklin D. Roosevelt addressed Congress and characterized December 7 as a "day of infamy," leading to an immediate declaration of war against Japan on December 8, 1941. This action effectively marked the official entry of the United States into World War II, as it not only engaged the U.S. in combat but also prompted Germany and Italy to declare war on the United States shortly thereafter, expanding the conflict further.

While the other factors such as the German invasion of Poland, support for the Allies, and the rise of Fascism in Europe were relevant to the broader context of the war, they each occurred prior to the U.S. entering the conflict. The direct and immediate cause of entry was the attack on Pearl Harbor, which unified the nation and shifted public opinion in favor of entering the war.