Army Group E
In response to the defeat of German forces in the Second Jassy–Kishinev offensive in late August 1944, which forced Bulgaria and Romania to switch sides, and to the advance of Red Army troops into the Balkans, Berlin ordered Army Group E to withdraw into Hungary. But the combined actions of Yugoslav partisans and Allied air forces impeded German movements with Ratweek. With the Red Army on Serbia's borders, the Wehrmacht put together another provisional army formation from available elements of Army Group E and the 2nd Panzer Army for the defense of Serbia, called "Army Group Serbia" ().