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No single event caused the United States to enter into World War I.
At the war’s outbreak in 1914, the U.S. government announced its neutrality. Public sentiment ran very strongly against U.S. involvement.
Germany's policy of unrestricted submarine warfare indeed became a major factor in drawing the U.S. into the war. The infamous U-boat sinking of the Lusitania, a British passenger ship, in 1915 began the slow shift of U.S. sentiment away from neutrality. Of the 1,198 passengers who died in the attack, 128 were American. The Lusitania was also carrying American-made ammunition that had been sold to the British military, a fact that was not disclosed to the American public. (The United States had begun to sell arms and munitions to the U.K.)
After the May 1916 U-boat sinking of the Sussex, a French passenger ferry, Germany's government promised that its U-boats would provide a warning and allow passengers to abandon ship safely before sinking nonmilitary vessels. The German military, however, eventually convinced government leaders to abandon the policy, and unannounced U-boat attacks began again. (A U-boat would make itself vulnerable when it surfaced to provide the warning.)
In his April 2, 1917, war message to Congress, Wilson declared:
The present German submarine warfare against commerce is a warfare against mankind.
It is a war against all nations. American ships have been sunk, American lives taken, in ways which it has stirred us very deeply to learn of. . . .
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