What caused the United States to become involved in World War 1?

Submarine warfare in the Atlantic kept tensions high, and Germany's sinking of the British ocean liner Lusitania on May 7, 1915, killed more than 120 U.S. citizens and provoked outrage in the U.S. In 1917, Germany's attacks on American ships and its attempts to meddle in U.S.-Mexican relations drew the U.S. into the war.

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