Why did the United States enter World War I?  

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The United States entered World War I (1914–1918) despite a long history of isolationism. Since its inception, America had stayed out of European wars, so why did it enter WWI in 1917? There are three reasons—one primary and two secondary—why the US joined the conflict: Germany's unrestricted submarine warfare, the Zimmermann Telegram, and President Woodrow Wilson's idealism.

After the commencement of hostilities in 1914, America attempted to trade with both sides. But the British had other ideas. Britain had the world's most powerful navy, and they blockaded Germany. Germany responded to this threat with submarine warfare. Because of the British blockade, American trade with the Allies increased as its commerce with Germany shrunk. In 1915, the Germans sank the Lusitania, killing over one thousand civilians. The Lusitania was a passenger ship that was carrying ammunition for the Allies. In fact, Germany had warned civilians not to travel on ships carrying weapons. In...

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