“How could the armistice after WW I have been any more unstable?”
The more interesting question is, “How could the end of the Great War have resulted in a more stable outcome if the US had stayed out?”
With a stalemate rather than the rout enabled by US intervention, the British Empire and the French would not have been in a position to carve up the Middle East and impose a punitive reparations regime on a prostrate German state and collapsing Austro-Hungarian empire, a settlement whose dire effects JM Keyenes accurately assessed in 1919.
http://en.wikipedia.org/wiki/The_Economic_Consequences_of_the_Peace
I can say with full belief, that our entrance into the war indirectly caused the second WW to be inevitable. Since we did fight that second war, our Marshall plan was also had within it the fertile seeds of socialist takeovers of the European countries. We left them in a position to being our dependents. That dependency on our protection, and the willingness of their people to allow another to rebuild them, allowed them to spend many millions on social programs. This directly led to the current collapse of the socialist dream. They didn’t have to worry about national defense, so we subsidized their own dependency on government.