Original: 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.
Simplified: 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.