For the campaign against the Soviet Union, the Germans allotted three army groups, totalling approximately 3.3 million men, along with 1 million from other Axis countries. Among these were 19 panzer divisions, and in total the “Barbarossa” force had about 3,000 tanks, 7,000 artillery pieces, and 2,500 aircraft. It was in effect the largest and most powerful invasion force in human history. Their Soviet opponents had vast amounts of equipment, but much of it was obsolete and poorly maintained and troop training levels were inadequate. In addition, a drastic purge in 1937 had crippled the Red Army, reducing its morale and efficiency just before the German invasion. With up to 50% of army officers executed, the result was that the Red Army officer corps in 1941 had many inexperienced senior officers. The battle of Greece and the invasion of Yugoslavia delayed the German invasion of the Soviet Union by a critical six weeks, but on June 22, 1941, Germany launched its invasion of the Soviet Union.