Been reading about WWI recently?
Anyways, my professor recently mentioned that "most people" around this time expected the war to last until 1919; the collapse of the German army in the fall of 1918 was unexpected. He stated that America's entry into the war rejuvenated the western allies, who were on there last legs at this point, and had the opposite effect in the German ranks. I know there are a lot of people (particularly British and French people

) who would disagree with that characterization of the USA's importance, but I think it's valid.
So let's think about those "America doesn't get involved" timelines. Say unrestricted submarine warfare is stopped after the British start convoying, and America backs down from threatening war. In that scenario I could easily see the war lasting through 1919. The main hurdle is finding a way to keep the German people from revolting due to food shortages. But extending the war through 1919 is certainly feasible.