Final answer:
The United States entered World War II in 1941 due to Japan's attack on Pearl Harbor, Roosevelt's desire to support the British and protect American interests, and the long-standing issues between Japan and the United States in the Pacific.
Step-by-step explanation:
The United States entered World War II in 1941 due to several reasons.
- The immediate cause was Japan's attack on the U.S. naval base at Pearl Harbor.
- President Roosevelt had been looking for ways to help the British fend off a potential German invasion, and Japan's aggression threatened the stability in the Pacific, which impacted American interests.
- There were long-standing issues between Japan and the United States regarding power in the Pacific, including Japan's expansion into Southeast Asia, which the U.S. wanted to stop.