Germany violated the Treaty of Versailles which was numerous and flagrant. The most significant violation was the rearmament of Germany, which was forbidden by the treaty. Hitler saw the limitation of the German military as a humiliation and a threat to his ambitions of conquest, so he began a massive rearmament program, including the development of tanks, airplanes, and other weapons. This move violated not only the spirit but also the letter of the treaty.
In addition to rearmament, Hitler sent German troops into the Rhineland in 1936, which was a demilitarized zone established by the treaty. This was a risky move, as France and Britain could have responded with force, but they did not want to provoke another war. Hitler interpreted their inaction as a sign of weakness and went on to annex Austria in 1938, which was another violation of the treaty. Finally, in 1939, Hitler demanded the annexation of the Sudetenland, a region in Czechoslovakia with a large ethnic German population. When the Czech government refused, Hitler invaded and occupied the country, which was the last straw for Britain and France. They declared war on Germany on September 3, 1939, and World War II began.
In summary, Hitler violated the Treaty of Versailles was a significant factor in the outbreak of World War II. His rearmament of Germany, occupation of the Rhineland, annexation of Austria, and aggression towards Czechoslovakia were all clear violations of the treaty's terms, which led to the appeasement of Hitler by other European powers, allowing him to continue his aggressive policies until it was too late.