Short answers, pulled from the story.
The Treveri, the Eburones, and other Celtic tribes occupied the territories between the Ardennes and the Rhine. These groups lived alongside Germanic neighbors who modified their cultures over time.
By the end of the 5th century the Franks had conquered all lands that had formerly been under Roman influence. By the 8th century Frankish dominion was firmly established in western Germania and northern Gaul.
At the Peace of Basel in 1795 France took the whole of the left bank of the Rhine. The population numbered about 1.6 million people living in numerous small states.
The Congress of Vienna expelled the French and assigned the lower Rhenish districts to Prussia. In 1822 the Prussian administration reorganized the territory as the Rhine Province.
On the 7th of March 1936 German troops marched into the Rhineland in violation of the Treaty of Versailles. There was no opposition to its re-occupation from other powers such as France and the UK.