Final answer:
Belgium controlled the smallest region of Germany post-World War I, as Germany lost territory to surrounding nations and had its military capabilities severely restricted.
Step-by-step explanation:
The country which controlled the smallest region of Germany after World War I was Belgium. Germany faced significant territorial losses following the war, as outlined in the Treaty of Versailles, which led to the loss of 13 percent of its European territory. Key regions were transferred to neighboring countries; for instance, Alsace-Lorraine was returned to France, while Belgium received smaller western territories. In the east, German lands were ceded to create the Polish Corridor, and the port of Danzig was made a free city. Furthermore, Germany surrendered its overseas colonies, like German New Guinea, which was delegated to Australian control post-war.
It's important to note that after World War I, Germany was not only territorially truncated but also demilitarized. The Treaty of Versailles imposed strict limitations on Germany's military capabilities. Key aspects of the treaty included the demilitarization of the Rhineland and significant reductions in the German army, navy, and the elimination of its air force.