Why did the United states enter WWI?

The most common explanation for the US entry into WWI emphasizes that the US entered the war because of Germany’s policy of unlimited submarine warfare.  The US disapproved of Germany’s submarine warfare because it seemed cruel and because it was contrary to international laws that were supposed to guarantee freedom of the seas.  The US had been outraged, for example, when a German submarine sank the passenger liner Lusitania with over 1,000 civilians (among who were more than 100 Americans) onboard.  The Germans stopped engaging in this sort of tactic for a while in response to American criticism.
When Germany resumed unrestricted submarine warfare in 1917, US public opinion turned more strongly against the Germans.  This hostility was exacerbated by the revelation of the Zimmermann Telegram, in which the Germans tried to get Mexico to enter the war on the German side in exchange for which the Germans would defeat the US and give Mexico back the land the US had taken from it in the Mexican-American war.  This outraged Americans who felt that it was wrong for Germany to try to incite the US’s neighbor to go to war and who felt that it was wrong for Germany to promise to take away US territory.
Historians, then, tend to say that the US entered WWI because of its opposition to unrestricted submarine warfare and because of the Zimmermann Telegram.
 
https://history.state.gov/milestones/1914-1920/wwi