Region of Germany. It is located west of the Rhine River and encompasses the states of Saarland and Rhineland-Palantinate and portions of Baden-Württemberg, Hesse, and North Rhine–Westphalia. The chief city of the Rhineland is Cologne. In the 19th century the Rhineland became the most prosperous area of Germany. After World War I, Allied troops occupied portions of the area on the border with France, and it was the scene of recurrent crises and controversies during the 1920s. In 1936 Adolf Hitler ordered German troops to enter the demilitarized zone of the Rhineland; weak objections by the Allies foreshadowed Hitler's later annexation of the Sudetenland.