The Allies all blamed Germany for the war even occuring, which was kind of stupid, considering it was Serbia that started the war, by assassinating Archduke Franz Ferdinand, heir to the Austria-Hungarian throne.
The Treaty of Versailles, the peace treaty that ended WWI, was specifically designed to punish Germany:
Under the Treaty, Germany:
Couldn't have an air force, their army was significantly reduced, their overseas colonies were taken by the Allies, and (most significantly), Germany had to accept complete responsibility for the war and pay large sums of money to the Allied countries.
Basically, Germany was a really bad place to live post-WWI, and the terrible economy and the German people's unhappiness helped Hitler come to power twenty years later.