Short answers, pulled from the story.
The Rhineland is a loosely defined area of western Germany along the Rhine, chiefly its middle section, and it sometimes extends into parts of Belgium. It has historic ties to the Holy Roman Empire, Prussia, and the German Empire.
Germany remilitarized the Rhineland on the 7th of March 1936, sending troops into territory the Treaty of Versailles had ordered kept free of German forces. France and the UK took no action to stop it, and the episode helped push Hitler toward the rearmament that led to war in 1939.
The Rhineland lies along the Rhine in western Germany, stretching west to the borders with Luxembourg, Belgium, and the Netherlands. To the east it reaches the Bergisches Land and the edges of the Westphalian and Hessian regions, and to the south it extends to the North Palatine Uplands.
At the Peace of Basel in 1795, France took the whole of the Rhine's left bank, home to about 1.6 million people across numerous small states. In 1806, the Rhenish princes joined the Confederation of the Rhine, a political creation controlled by Napoleon.
In 1946 the Rhineland was divided among the newly founded states of Hesse, North Rhine-Westphalia, and Rhineland-Palatinate. North Rhine-Westphalia became a major German industrial area with deposits of coal, lead, lignite, magnesium, oil, and uranium, while Rhineland-Palatinate is known more for agriculture, including vineyards along the Ahr, the Mittelrhein, and the Mosel.
Before Roman rule, the territory between the Ardennes and the Rhine was occupied by Celtic tribes, including the Treveri and the Eburones, both shaped by their Germanic neighbors. On the Rhine's east bank, the Mattiaci, a branch of the Germanic Chatti, settled between the Main and Lahn rivers, while the Usipetes and Tencteri lived farther north.