July 1944 Issay-Kishinev Soviet Operation to Surround the Reformed German 6th Army was followed by Rumania Declaration of War vs Germany,
Leaving Bulgaria, where there were no German Troops, Creating a Further considerable Gap along the South Sector of East Front.
In Rumania the German Line was withdrawn toward Hungary.
August -Sept Soviet Operations were expedited on Stalins orders to Sweep across Bulgaria to secure this open area for OPerations in Yugoslavia then North toward Belgrade