The East Cantons were part of the Rhine Province of Prussia in Germany until 1920 but were annexed by Belgium following Germany 's defeat in World War I and the subsequent Treaty of Versailles . The East Cantons were part of the Rhine Province of Prussia in Germany until 1920 ( as the counties ( Landkreise ) of Eupen and Malmedy ) , but were annexed by Belgium following Germany 's defeat in World War I and the subsequent Treaty of Versailles .