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How did the Rhineland become a demilitarized zone after World War I?

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Treaty of Versailles The “Big Four” (left to right): David Lloyd George of Britain, Vittorio Orlando of Italy, Georges Clemenceau of France, and Woodrow Wilson of the United States, the principal architects of the Treaty of Versailles.

Following World War I, the Treaty of Versailles established the Rhineland as a demilitarized zone to provide France with a "material barrier" against future German aggression.12

This status was achieved through the following measures:

Permanent Demilitarization: The treaty mandated that the entire German territory on the left bank of the Rhine, along with a 50-kilometer (approximately 30-mile) wide strip on the right bank, be permanently demilitarized.12
Allied Occupation: To ensure compliance and provide security, the treaty allowed Allied troops to occupy portions of the left and right banks of the Rhineland. This occupation was organized into three zones, with bridgeheads at Cologne, Koblenz, Mainz, and Kehl, scheduled for serial evacuation at five-year intervals.12

These provisions were the result of a compromise between French demands for the permanent separation or annexation of the Rhineland and the opposition of British and American leaders, who feared that dismembering Germany would sow the seeds of future conflict. The last of the Allied occupying troops eventually departed the region on June 30, 1930.21