What caused the US to enter war world one?

Answer 1

There were a number of "casus belli" (Latin for "Causes of War"), but the ultimate reasons were based on Germany's policy of unrestricted submarine warfare.

The "Zimmerman telegram", a German diplomatic message addressed to Mexico, had proposed that Germany and Mexico should enter into an alliance in which previously Mexican territories such as Texas, New Mexico and Arizona would be restored to Mexico when Germany won the war. Mexico ignored the proposal, but the message was intercepted and decrypted by the British, who used it encourage the US to enter the war overtly on the side of Britain. This provided the final impetus for Congress to declare war.