German troops re-occupied the Rhineland, a de-militarized zone in Germany according to the Treaty of Versailles that bordered on France. This action was directly against the terms which Germany had accepted after the First World War.
Hitler argue that it was done in response to France and the USSR signing a treaty of friendship and mutual support, saying it was a hostile move against Germany, and the area of the Rhineland could in turn be used by France to invade Germany. More