Un-restricted submarine warfare. If it wasn't for that, the US would not have entered the war. As it was, the US was only in it for a year anyway...it ended in 1918 (the US entered the war in 1917).
The US became a combatant, part of the allies in 1917.
1917
1917
1941
1941
1914
1917
on decmber 8 1941
The US enters WW1 in 1917
1917
Knowing which war is being asked about might help in answering the question.