You mean in practice or in theory? Because with any political party, the outcome is always different.
Their entry into the war boosted the morale of the other allied nations. Morale completely depleted seems how the war was ultimately a stalemate and lives were being lost needlessly. American entry into the war also discouraged the Axis powers, and later after America's entry the Axis powers began to drop deeper into fear. America was the turning point for the war. The Axis powers were winning and if it weren't for America, then the Axis powers would have prevailed and the world would not be the way it is today. (This is what I think about it, it might not be entirely correct.)
