What started World War 1 for America?

Answer 1

The combination of three main events, that were; Unrestricted Submarine warfare from Germany's behalf, the zimmerman telegraph, which was an incident where Germany sent a telegraph to Mexico asking them to deaclare war upon the U.S. and the sinking of the Lusitania in 1915 killing 128 U.S. and an additional 4 U.S. passanger ships carring supplies to Britain for the war effort.

For these reasons the U.S. based their grounds to start war, and declar war against Germany.