Overview:Part 1 of 3. President Woodrow Wilson vowed to keep the U.S. out of World War I after hostilities erupted in Europe in August 1914. It was a promise he kept until 1917, when the Germans resumed "unrestricted submarine warfare"— a policy it had started and then stopped in 1915 — and began sinking U.S. ships. An intercepted telegram also showed Germany trying to convince Japan and Mexico to declare war on America.