Why did this attitude of an unfair peace and the US noninvolvement in the league of Nations help set...?

The era after world war i was still an era of greed and power mongering, where all countries who won and lost still were thinking of economic advancement, and were not afraid to deploy a sacrificial lamb (germany and her allies) so as to reach this goal.

It is all a mess and the US should look in instead of out.

RE:

Why did this attitude of an unfair peace and the US noninvolvement in the league of Nations help set...?

Because the Germans were left to the (lack of) mercy of the French and the Brits. It was very much like American Reconstruction after our Civil War. It destroyed the economy of the region. In the Southern US reconstruction started groups like the KKK and in Germany after WWI, they had only three choices... Starve to death, go Communist, or go Nazi.

Now, today, the Brits blame the Americans for not "getting into WWII faster", even though Both France and Britain sat and watched Germany re-arm, and allowed the Soviets to steal more land than the Germans did and only complain about the Germans. Of course, without the Food from the US, the Brits and French would be speaking with a heavy German accent today if they spoke French and English at all.

As far as the US not getting into the League of Nations, you have only to look at what the Brits and the French said about the US during the treaty that ended WWI. The US, who had sent their Head of STATE over to the treaty talks, was insulted, slandered and treated like we were one of their colonies again. We basically told them to go to hell, even though our weak-kneed president tried to get us to join that circus.