The argument for Soviet adherence to the Molotov-Ribbentrop arrangement rests on defensive depth. If Soviet forces failed to take the territory allocated to them, Germany could occupy it instead. Continuing with the agreement therefore preserved defensive depth between the Soviet Union and Germany.
Notable Point · 1:18:36-1:19:17
Soviet defensive depth and the Molotov-Ribbentrop Pact
The Soviet incentive to retain the pact is explained through the territory separating Soviet forces from Germany.