Complex sentence: In 1946, the administration of the French zone of occupied Germany de facto annexed an area of in the German part of the Upper Mundat Forest, in order to guarantee the water catchment area for Wissembourg.
Split into simpler sentences: In 1946, the administration of the French zone of occupied Germany made an area of in the German part of the Upper Mundat Forest administratively a part of France. This was in order to guarantee the water catchment area for Wissembourg.