Maybe France somehow becomes embroiled in another violent revolution, this one communist in nature, and Germany reoccupies the Rhineland/Saarland when French troops are recalled by the government to fight the civil war. Later, the Germans occupy the city of Strasbourg to keep the peace when a riot breaks out, with the French military police in the area throwing down their arms and mutinying. Germany de facto administers Alsace-Lorraine until the situation settles, at which point it is incorporated into the Wiemar Republic.