What was the main reason the United States entered World War II?

Why This Matters

The main reason the United States entered World War II was the Japanese attack on Pearl Harbor on December 7, 1941. Before this attack, the United States had been trying to stay out of the war, even as conflicts raged across Europe and Asia. The surprise bombing of the American naval base at Pearl Harbor in Hawaii changed everything overnight.

On that Sunday morning, Japanese planes attacked without warning, destroying or damaging 19 American ships and over 300 airplanes. More than 2,400 Americans were killed and over 1,000 were wounded. The attack shocked the entire nation. President Franklin D. Roosevelt called December 7th "a date which will live in infamy" and asked Congress to declare war on Japan the very next day. Congress approved the declaration with only one dissenting vote.

Within days, Japan's allies, Germany and Italy, declared war on the United States, pulling America fully into the conflict in both the Pacific and Europe. The U.S. went on to play a central role in defeating the Axis powers. The war ended in 1945 with victories in both Europe and the Pacific. Pearl Harbor remains one of the most significant moments in American history because it transformed the country from a reluctant observer into the world's leading military power.

Key Facts

• Pearl Harbor was attacked on December 7, 1941
• The attack was carried out by the Japanese military
• Over 2,400 Americans were killed in the surprise attack
• President Roosevelt asked Congress to declare war the next day
• The U.S. fought in both the Pacific and European theaters of World War II

Common Mistakes

• Confusing Pearl Harbor with reasons for entering World War I, WWI was about submarine warfare and the Zimmermann Telegram
• Saying the U.S. entered WWII because of events in Europe, while Europe was at war, it was the Pacific attack on Pearl Harbor that directly triggered U.S. involvement
• Getting the date wrong, remember December 7, 1941, not 1942 or 1940

Study Tip

This is one of the most straightforward answers on the test. If the question mentions World War II, think "Pearl Harbor." President Roosevelt's famous phrase "a date which will live in infamy" can help you remember both the event and its importance. Practice saying "The Japanese attacked Pearl Harbor" as your answer.