Option 4: Germany's declaration of unlimited submarine warfare
Germany's announcement to their policy of unrestricted submarine warfare in the North Atlantic and the Mediterranean on February 1, 1917, led the American President Woodrow Wilson to publicly declare war against Germans two days later, and just within hours of the announcement, a private American cargo ship, Housatonic, was torpedoed by a German submarine. Fortunately, no one was onboard.
Before formal German's declaration, during the war, the nation had already broken its promises to restrict submarine warfare and had caused the sinking of several U.S. ships before, including the Lusitania, in May 1915. Germany apologized every time, but its actions turned Americans public opinion against Germany. Their 1917 announcement was enough for the U.S. to finally enter WW I in April on the side of the Allies.
