Answer:
The Soviet's "secret advantage" over Germany during the Battle of Stalingrad was their superior knowledge of the terrain and weather conditions in the region, as well as their ability to draw on reinforcements and supplies from across the vast Soviet Union. The German army, on the other hand, was stretched thin and ill-prepared for the harsh winter conditions that set in during the battle. Additionally, the Soviet army had a strong sense of determination and patriotism, which helped them to endure and ultimately defeat the German forces.