In June 1941, Nazi Germany launched an attack upon the Soviet Union. The German Reich had been building up forces along the Eastern Front for a long time, but the sudden aggression caught Soviet forces unprepared. Many troops were captured and the Germans quickly conquered territory from the Soviet states. But the Soviets reorganized, improved their communication structure, and pulled together a defense at Smolensk. Although they lost again, they critically slowed the German advance and halted their race towards Moscow. Instead, the Germans tried to lay siege at Leningrad, only to be struck themselves by insufficient supply lines and a brutal winter that claimed the lives of many soldiers. With that, the Wehrmacht withdrew and redirected its efforts towards Stalingrad. Josef Stalin refused to let them take any land "further than the Volga" in Russia, and mounted a stiff defense. Even when the Luftwaffe, the German air force, reduced the city to rubble, Soviet soldiers continued to wage war from the debris. Meanwhile, the Germans were so focused on their offensive that they let their defensive lines collapse, and in October 1941 the Soviets managed to surround and pin down the German 6th Army. Their commander refused surrender terms because he didn't want to displease Adolf Hitler, but the 6th Army's resistance inevitably collapsed in February 1942.
