Harsh Treaty of Versailles, massive war debt, and a weak League of Nations
The Treaty of Versailles was very harsh toward the losing side in particular Germany. The harshness of the treaty caused nationalistic reactions leading to military buildup and extreme leaders to take control.
Massive debt in Europe led to depression in Europe and eventually in the US. The depression made economies stop dead in their tracks and caused hyperinflation in Germany as well as weak currency. These conditions also contributed extreme leaders.
Lastly, the construction of the League of Nations provided weak leadership when countries became aggressive. The US refused to join despite being the architect of the organization. In addition, the countries turned to appeasement instead of addressing the aggressors.
