In June 1941, Germany invaded the Soviet Union, initiating the war on the Eastern Front. Although intelligence agencies had repeatedly warned him of Germany's intentions, Stalin was taken by surprise. He formed a State Defense Committee, which he headed as Supreme Commander, as well as a military Supreme Command (Stavka), with Georgy Zhukov as its Chief of Staff. The German tactic of "blitzkrieg" was initially highly effective; the Soviet air force in the western borderlands was destroyed within two days. The German Wehrmacht pushed deep into Soviet territory; soon, Ukraine, Byelorussia, and the Baltic states were under German occupation, and Leningrad was under siege; and Soviet refugees were flooding into Moscow and surrounding cities. By July, Germany's Luftwaffe was bombing Moscow, and by October the Wehrmacht was amassing for a full assault on the capital. Plans were made for the Soviet government to evacuate to Kuibyshev, although Stalin decided to remain in Moscow, believing his flight would damage troop morale. The German advance on Moscow was halted after two months of battle in increasingly harsh weather conditions.