At the beginning of German invasion of the USSR, most of the Soviet Union's tank force was composed of the T-26 and BT tanks series. A few T-40s had also appeared, along with about 1,363 mechanically unreliable early-model T-34 tanks and 677 KV series tanks. The early-model T-34s' heavy losses were caused by lack of coordination, lack of supplies, bad training, mechanical issues and the Red Army's general lack of preparation for war. Another difficulty for the T-34 was that it had only a four-man crew, with the tank commander forced to double as the gunner. Although he was spared from loading duties, unlike commanders in French tanks, it still crippled the tank commander's ability to maintain awareness of the battlefield, which gave German armour a tactical advantage.