Well WW1 was quite an odd war in that all the belligerents (countries fighting each other) had monarchs who were all related to one another, and so the United states saw the war as a European war, or even a family spat, with no need to enter on either side. It was only when a German U-Boat sunk the "Lusitania", a US cruise boat carrying nearly 1,200 people, of whom most were American, that the US entered the war.
Hope I helped :)
