Why did the Americans join war world 1?

The Germans wanted the Mexicans to attack USA to avoid their intervention but the Americans found out about it and interviened.

Well because after Germany warned the US not to send any ships in European waters and warned American not to board any vessels in the area Americans chose to board the Lucitania anyway and the us wanted in the war so when the Lucitania was bombed that was our excuse to enter the war.