Why did the United States enter World War II?

The entry of the United States into World War II is most directly linked to the attack on Pearl Harbor by Japan on December 7, 1941. This surprise military strike led to significant loss of life and damage to the U.S. Pacific Fleet, prompting a strong national response. Following the attack, President Franklin D. Roosevelt addressed Congress, describing the day as "a date which will live in infamy," and called for a declaration of war against Japan. This event galvanized public opinion and united the nation in support of the war effort, marking a clear turning point where neutrality was abandoned and military engagement was deemed necessary for national defense.

The other options, while related to the broader context of the war, do not directly capture the immediate cause for the U.S. entering the conflict. Supporting allies, opposing fascism, and preventing nuclear warfare involve longer-term motivations and outcomes of the war rather than the initial catalyst that led to U.S. involvement. The attack on Pearl Harbor was a pivotal moment that spurred the U.S. into direct conflict, highlighting its significance in American history.