Answer:
Hitler "remilitarized" the Rhineland in 1935. The Rhineland is that portion of Germany west of the Rhine River, on the same side as France. Under the terms of the Versailles Treaty the Rhineland was to be "demilitarized" - no troops could be based there, no fortifications built, and so on. Hitler also restarted the German Air Force in 1935, another thing forbidden by the Treaty of Versailles. At this point German rearming had not progressed very far, and had the French and British acted to force Hitler to abide by the terms of the Versailles Treaty, the war might have been averted. But they did nothing more than protest.