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U.S. Involvement in World War II: Causes, Impact & Timeline

Lesson Transcript
Instructor: Nate Sullivan

Nate Sullivan holds a M.A. in History and a M.Ed. He is an adjunct history professor, middle school history teacher, and freelance writer.

In this lesson, learn about American involvement in World War II. We'll learn how the Japanese attack on Pearl Harbor brought the U.S. into the war. We'll also highlight key developments throughout the war and discover its long-term impact. Updated: 04/14/2021

War on an Unprecedented Scale

Imagine waking up on the island of Hawaii on Sunday morning, December 7th, 1941. You're having breakfast or getting ready for church on a day that seems like any other. Then suddenly you hear the roar of fighter planes in the skies. This is no drill! The Japanese are attacking Pearl Harbor!

For many Americans, this was a real-life experience. World War II was an unbelievably tragic event in human history, taking the lives of somewhere between 50 to 80 million people throughout the world. While the death toll among Americans was relatively low compared with other countries (roughly half a million people), the war had a profound impact on the United States.

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  • 0:04 War on an Unprecedented Scale
  • 0:48 Pearl Harbor
  • 2:07 U.S. Involvement in…
  • 4:40 U.S. Involvement in…
  • 5:37 Lesson Summary
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Pearl Harbor

World War II began in 1939 when Nazi Germany invaded Poland. Great Britain and France game to Poland's defense, while Fascist Italy allied itself with Hitler's Germany. At this point, the U.S. was not directly involved in the war, although it did support Great Britain by sending supplies to the beleaguered island nation. Some Americans felt the U.S. should become involved, but isolationism tended to be more popular. Isolationism is the foreign policy approach of avoiding involvement in the affairs or conflicts of other nations.

This all changed on the morning of December 7, 1941. The Japanese had been an ally of Nazi Germany and Fascist Italy and were seeking control of the Pacific Ocean. They believed they could destroy the American Navy in a surprise attack from which the U.S. could not recover. The Japanese attack on Pearl Harbor devastated the American Navy, sinking four battleships, damaging the rest, and killing 2,403 Americans. The following day, President Franklin D. Roosevelt delivered a speech to Congress, asking for a declaration of war. The U.S. was a ''Sleeping Giant'' that the Japanese had not killed off, but merely awoken.

U.S. Involvement in WWII: Timeline

Let's look at some of the most important events of World War II in relation to U.S. involvement (we've already discussed the significance of Pearl Harbor). We've already discussed the significance of Pearl Harbor.

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