Answer: War guilt
Explanation: The treaty of Versailles made it so Germany took sole fault for the war and had to suffer demilitarization, reparations to the Allied forces and loss of territory alongside that. Germany had lost about 13% of its territory and also had to pay somewhere around 33 Billion US dollars to cover civilian damages. Germany's army was diminished to 100,000 soldiers and was forbidden to draft soldiers again. Which basically kept them at bay, preventing any further aggression from them.