Use of armoured forces was crucial for both sides on the Eastern Front. Operation Barbarossa, the German invasion of the Soviet Union in 1941, involved a number of breakthroughs and encirclements by motorised forces. Its goal according to Führer Directive 21 (18 December 1940) was "to destroy the Russian forces deployed in the West and to prevent their escape into the wide-open spaces of Russia." The Red Army was to be destroyed west of the Dvina and Dnieper rivers, which were about east of the Soviet border, to be followed by a mopping-up operation. The surprise attack resulted in the near annihilation of the Voyenno-Vozdushnye Sily (VVS, Soviet Air Force) by simultaneous attacks on airfields, allowing the Luftwaffe to achieve total air supremacy over all the battlefields within the first week. On the ground, four German panzer groups outflanked and encircled disorganised Red Army units, while the marching infantry completed the encirclements and defeated the trapped forces. In late July, after 2nd Panzer Group (commanded by Guderian) captured the watersheds of the Dvina and Dnieper rivers near Smolensk, the panzers had to defend the encirclement, because the marching infantry divisions were still hundreds of kilometres to the west.