(Answer #1). The US entered World War I largely because of the German practice of submarine warfare. The entry of the US broke the stalemate that had existed for most of the war and led to the Allied victory.The US did not like the fact that Germany was using submarines to sink ships headed for England.he effects of the US entry are likely even harder to pin down as the Allies already had quite an advantage over the beleaguered German and Austrian forces and they were likely to sue for peace before much longer though there is some support for the idea that the American entry into the war helped to shorten it.