Hitler was able to build up the German army despite Treaty of Versailles restrictions by taking advantage of appeasement and a lack of force behind the League of Nations. This allowed him to gradually increase the size and strength of the German military beyond what was permitted by the treaty. Appeasement, which involved giving in to Hitler's demands to avoid conflict, provided him with the opportunity to rearm Germany without facing significant opposition from other countries.
Additionally, the League of Nations, which was established to promote peace and prevent future conflicts, lacked the ability to enforce its decisions effectively. This weakness further enabled Hitler to strengthen the German army without facing immediate repercussions.
By exploiting these factors, Hitler was able to build up the German army in violation of the Treaty of Versailles, ultimately leading to the outbreak of World War II.