The event that finally caused to ask Woodrow Wilson for the declaration of war against Germany was the announcement of a policy of unrestricted submarine warfare by Germany on February 1, 1917, and the sinking of the American cargo ship Housatonic on February 3, despite their previous promises of leaving American ships alone.
By that time Wilson had already decided to break relationships with Germany, and such decision was reaffirmed when, days later, the U.S. received the Zimmermann Telegram, a secret message decoded by Britain, in which the German foreign secretary Arthur Zimmermann proposed Mexico an alliance in the case of war between the U.S. and Germany, promising Mexico financial and territorial rewards for its support.