I know that U.S. economic involvement is widely regarded as substantial and beneficial in helping the Allied powers, but I'm curious if their military involvement is as significant. Since they were involved for a relatively short amount of time before the end of the war, is it possible that the war could have ended without their help?
Well, IIRC US involvement in the war convinced the Germans to stop fighting, because of overwhelming superiority, and also convinced the allies to try and end the war quickly. Basically, if the war had continued into 1919, the US would have been able to dictate to its allies almost as much as it would have been able to dictate to the defeated Germans.
I'm sorry, I can't recall the source of that! But any decent book on the end of the First World War, or indeed the origins of the Second, should tell you something similar :)