Answer: they feared an attack from the allied forces.
The Treaty of Versailles was the treaty that brought WWI to an end. It was signed on June 28, 1919 in Versailles, five years after the assassination of Archduke Franz Ferdinand.
One of the most controversial provisions in the treaty was the "War Guilt Clause" which required Germany to accept responsibility for the loss and damage of the war, and pay reparations.