The United States entered World War I principallyÂ in response to Germany's continued submarine operations in the North Atlantic and Mediterranean, which it had earlier pledged to cease. Another reason was Germany's attempt to ally with Mexico against the United States.Â President Woodrow Wilson presented both of these reasons before a joint session of Congress on April 2, 1917, and war was declared on Austria-Hungary on December 7, 1917.
Particularly troubling to the United States about Germany's submarine operations were their strikes on non-military (passenger and merchant) ships. Despite agreeing to Woodrow Wilson's "Sussex Pledge" (named after an unarmed French vessel sunk by the Germans), Germany resumed unrestricted submarine warfare as a means to quickly defeat British forces.
Some in Germany, such as the Chancellor Theobald von Bethmann-Hollweg, feared the entry of the United States into the war. However, it was the prevailing attitude within the German government that continued submarine operations, even in defiance of the Sussex Pledge, could ensure their victory long before the arrival of the United States' forces.
The fact, which was revealed by British naval intelligence, that Germany was also attempting to ally with Mexico, promising to return land stolen by the United States in the Mexican-American War, was the deciding factor for Wilson to declare war.