1) The first and main reason America entered WW2 because of the "Zimmerman telegram," intercepted by the British, in which Germany floated the idea of an alliance with Mexico.
2) In May 1915, a German U-boat sunk the British passenger ship Lusitania off the coast of Ireland. Over 1,000 passengers were killed, including 128 Americans after it was sunken by the Germans.
3) Germany turned to unrestricted submarine warfare to keep goods from reaching Britain.