The Treaty of Versailles
The Treaty of Versailles was basically a way for the Allied Powers to take out their anger and frustration on Germany without really thinking about the long term, or even the short term consequences. What was expected from Germany was not only completely unfair but it was also impossible for Germany to comply with the punishments against it. The Treaty completely disregarded the Fourteen Points that Woodrow Wilson negotiated as part of the armistice that was negotiated with Germany.
Germany was expected to almost completely disarm with its massive army being drastically reduced. The reparations were seen as too large for Germany to ever be able to pay, though some argue that with the reduced spending on the military Germany could have paid the reparations. However, it would have still stagnated the Germany economy as they would not be able to put that money toward their industries. Another issue of the Treaty of Versailles was that while it did free the countries that had been occupied by Germany it gave no thought to the Germans within those countries. Many of them were brutally harassed and victimized causing them to not only flee their homes but harbor resentment toward the Allied powers.
The Treaty also caused resentment throughout Germany. The Allied Powers forced Germany to take sole responsibility for the war and put Germany in a position where it would not be allowed to have any sort of positive influence within Europe. This coupled with the economic depression of the 1930s fostered huge hatred within Germany for the Allied Powers and a desperate desire to blame someone for the dire straits that Germans now found themselves in. This not only allowed for socialism to grow but for Hitler to come to power and build the military back up to try and conquer Europe again.