Analysis of information sources in references of the Wikipedia article "Stay-behind" in Portuguese language version.
The Pripet Swamp, which created a great gap between German Army Group Center and Army Group North soon after ... June 1941, made it impossible for large military formations to conduct mutually supporting operations. Attempts to bypass such extensive wetlands proved perilous, because outflanked Soviet stay-behind forces and partisans pounced on logistical troops as soon as German spearheads disappeared.
The Finns soon became seriously hindered and harassed by Soviet forces operating in their rear areas. Some of these units were left to operate as stay-behind or partisan units as the Soviets retreated.
