Hitler always considered his 1939 non-aggression pact with Stalin to be temporary. After dividing Poland with the Soviet Union, he soon began planning his next eastward assault. Defeating Communism was a priority for Hitler on par with exterminating the Jews.
Under the codename “operation Barbarossa”, with a massive force of over three million soldiers, the Nazis invaded the Soviet Union over a vast front on June 22, 1941.
The Soviet army was caught by surprise and the Germans were able to advance quickly. Murderous mobile killing units, the Einstatzgruppen, accompanied the advancing armed forces, organizing the systematic murder of over one million Jews.
Despite terrible losses, the Soviet Union managed to hold off the German army during the winter of 1941-1942. The Germans, supported by Finnish, Hungarian, Romanian, Slovakian, Croatian and Italian forces, resumed their offensive in the summer of 1942, reaching the outskirts of Stalingrad (now Volgograd) in southeast European Russia The Battle of Stalingrad was one of the bloodiest in the history of warfare, with combined casualties estimated at about two million people. Although the Nazis at one point occupied almost 90% of the city, they could not dislodge the tenacious Red army. When the German Sixth Army was finally surrounded and surrendered in February 1943, it proved to be the turning point in the war.