Following the German decision to re-start unrestricted submarine warfare, which led to the sinking of neutral American ships, the American government, thanks to British intelligence, had come into possession of the infamous Zimmerman Telegram.
The Zimmerman Telegram was a coded signal sent from German High command to the government of Mexico in January 1917 by Arthur Zimmerman, the German Secretary for War, proposing a formal German-Mexican military alliance should the USA enter into WWI on the side of the Allies.
This proved too much for the Americans to stomach and on 4th April 1917, the US Senate voted by 82-6 to formally enter the Great War against Germany.
