I sounds like you're pretty well versed in your European history. So I had to go back and brush up a little. These types of questions are always interesting because one never really knows how cause and effect will play out. Even though you didn't tread in that arena, I agree that America-bashers use these parlor games to gratify their dislike for America.

As I recall President Wilson had a stroke during the negotiations of the Versailles Treaty. He did indeed want easier terms. And if I'm not mistaken, one of the reasons Germany's coup was successful is because the allies signaled they would deal fairly with Germany if she surrendered. The Germans had been at war for four long years and were ready to have done with it. Germany was also very fractured politically. There were communists, socialists, monarchists, democrats, and everything in between. So the opposition parties used the unpopular war to gain advantage over Kaiser and bring themselves to power.

(FEEL FREE TO DIFFER IF YOU THINK OTHERWISE)

So the new government in Germany abandoned the war effort expecting even handed treatment. The problem came when France undermined the treaty by playing politics and imposing harsher terms than America wanted. And, when Wilson became disabled, they used that to their full advantage. The French imposed reparations on Germany for the war. This caused economic turmoil in Germany. It also lead to the runaway inflation. As I recall there were items of contention about who controlled the assets in Alsace-Lorraine. Plus the area of the Ruhr Valley was operated by Germany but most of the output was taken as war reparations too.

So the Germans did have a point when they claimed they were cheated by the Treaty of Versailles. On the other hand, they were at war and expecting the French to play nice was very naive. So, now we come to your original question: What if America had stayed home? I think the Germans would not have had an excuse to abandon the war and the coup would not have happened. France and Germany would have been forced to negotiate. And as such, they would have come to more even terms. Reparations would not have been an issue and the depression would not have been or at least not have been as severe.

So, in some regard our intervention did more harm than good. We lost a lot of good men, and didn't really resolve anything. But who's to say they would not have gone to war again at a later date, but this time without the leadership of Adolf Hitler. They might have actually made some sane decisions at crucial times such as not attacking Russia. In times of defeat, a good leader would have saved his army rather than let it be annihilated at Stalingrad. A good leader would have sued England for peace right after Dunkirk and played on the anti-war sentiment in England at that time. They would have used the anti-war sentiment against the allies just as it was used against them in WWI.

Let me make a current observation based on a historical perspective. First, we see that meddling with good intentions doesn't always work. If we have faith in ourselves to be self-governing, we should have faith in others to make their own decisions. In that regard, we needed to restore some order to the Middle East for our own security. Our involvement in Afghanistan and Iraq was nothing like our involvement in WWI. But we see politicians using anti-war sentiment very effectively to gain political advantage. Unfortunately the citizenry doesn't have the historical perspective of some and can fall prey to an anti-war emotion appeal. We have to be prepared to see our actions through to a successful conclusion, whatever that is and however long it takes. If we don't, we risk fomenting future problems of those who distort history like Hitler did regarding the Versailles Treaty.

RJ