Answer:
The United States entered World War I primarily because of the increasingly aggressive German submarine campaign in the Atlantic Ocean. German activity was seriously disrupting US exports to Europe, which led American businessmen and industrialists to put pressure on the Wilson Administration to enter the war.
Another event that caused the United States to enter World War I was that in January of 1917, British intelligence would intercept the Zimmerman telegram. The Zimmerman telegram was a secret communication from Germany to Mexico, involving a potential alliance between the two countries should the United States enter the war.
These two events were critical in shaping American intervention, which allowed the United States to side with the Allies against the Central Powers.
