For many years the Rhineland area had been a key industrial region of Germany, producing coal, steel and iron resources. The Rhineland also formed a natural barrier to its neighbour and rival, France. In the event of a war, the River Rhine, if properly defended, would be a difficult obstacle for an invading force to cross.
But, in 1936, Hitler boldly marched 22,000 German troops into the Rhineland, in a direct contravention of the Treaty of Versailles