The United States finally decided to join the World War I on side of the Allies mainly because of the following 2 reasons:
* The first one was the proposal made to Mexico in order to join the war against the allies, which meant the war was getting closer to American land.
* The second one was the German submarine attacks on American merchant ships.
After these two events took place, Woodrow Wilson the American president at the time talked to the Congress and requested it to declare war on Germany. And finally the Congress declared the war on April 6th, 1917.
