If the U.S. hadn't entered World War I, would there have been a World War II?

We cannot know what impact continued U.S. neutrality would have had; by its nature, counter-factual history is a speculative enterprise. But we do know the consequences of the U.S. decision to join the Allied powers in the spring of 1917.

The American Expeditionary Force, commanded by General John Pershing, played a highly significant and, perhaps, decisive part in compelling the Germans to sue for peace in the fall of 1918. They were forced to accept a humiliating Armistice, in which the Kaiser abdicated and fled into exile. What turned the tide was not the actual fighting done by U.S. troops, nearly all of which occurred in the final six months of the conflict. German generals recognized they had no ability to counter the two million or more doughboys set to arrive in the summer of 1918. So, that spring, they threw all their remaining forces into a final offensive in northern France. We must strike, General Erich Lundendorff told his fellow commanders, “before America can throw strong forces into the scale.” That strike failed and, with it, any hope for some semblance of the “peace without victory” in which Woodrow Wilson devoutly believed.