With Germany in an economic crisis due to WW1, the people grew angry with their unfair treatment. Hitler convinced the Germans that they were not weak and that they should get revenge on the other countries.
Instead of following the Treaty of Versailles, Hitler chose to take lands he was told not to, like Poland, Czechoslovakia, and Austria. The Allies did not take any action.
He continued to sign treaties and not follow them, but the other nations wanted to avoid another war. That did not work.