During and after World War I, much of Europe experienced devastating losses, both of people killed in the war, and huge economic losses. Instead of helping create a balance of power in the region, the nations that "won" the war seemed to feel the need to punish the countries that lost. This created even worse conditions in those countries, notably Germany. This helped Hitler and the Nazi party come to power; things were so bad that it was much easier to convince the population that things would be better if they were in power, and that everything was the fault of the Jews. The intolerance had existed for a long time, but the situation after World War I made things much worse.
Because World War I led both directly and indirectly to poverty in European countries and elsewhere after the War, it also led to intolerance. There's a close link between the two.
When the German economy crashed, it caused widespread misery and economic dislocation. Unemployment, hunger, and civil strife became common, and crime rose. In conditions such as that it is easy to look for scapegoats - someone to blame - and Hitler and the Nazi Party, among others, sought to provide the public with that scapegoat. In that case it was Jews and Communists.
- Italy which was a part of the victor Allies group was not given its share in the fruits of victory.
- The terms of the post-war treaties was humiliating for Germany and its was given all the credit for the war losses.