After World War I, the French and the British, in particular, were eager to punish Germany and to ensure that it could never start another war in the way that it had (they said) caused WWI.
The Allies took many steps to weaken and punish Germany. They drastically reduced the size of the military that it was allowed to have. They took away its right to have most kinds of offensive weapons. They forced Germany to pay heavy reparations for war damage. They took away parts of Germany (including all of Germany’s overseas empire) and prevented Germany from keeping military forces in some areas that were left in German hands. Finally, they forced Germany to admit that the war was their fault.
All of this laid the foundation for World War II. Germans were extremely unhappy about what they saw as an unfair and excessively punitive treaty. This made them long for revenge. This longing for revenge helped to allow Hitler to come to power. In these ways, the Allies’ punishment of Germany for World War I had the paradoxical effect of helping to cause World War II.