It was 70 years ago today, on June 22, 1941, that Germany invaded its erstwhile ally the Soviet Union in World War II in an assault code named Operation Barbarossa. Up to that point, Adolf Hitler‘s Nazi German army enjoyed a string of almost unbroken military successes, but the failure of German troops to defeat Soviet forces signaled a crucial turning point in the war.

Until Barbarossa, Germany and the Soviet Union had a nonaggression pact, though that was largely for reasons of expediency, since Hitler harbored deep-seated feelings of anti-Bolshevism. Originally, the invasion of the Soviet Union had been scheduled for mid-May 1941, but it was postponed to allow for a German invasion of Yugoslavia and Greece, and it was originally code named Operation Fritz. Hitler changed the name to Barbarossa, after Holy Roman emperor Frederick Barbarossa (reigned 1152–90), who sought to establish German predominance in Europe.

As Britannica discusses,

For the campaign against the Soviet Union, the Germans allotted almost 150 divisions containing a total of about three million men. Among those units 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. The Germans’ strength was further increased by more than 30 divisions of Finnish and Romanian troops. The Soviet Union had twice or perhaps three times the number of both tanks and aircraft as the Germans had, but their aircraft were mostly obsolete. The Soviet tanks were about equal to those of the Germans, however. A greater hindrance to Hitler’s chances of victory was that the German intelligence service underestimated the troop reserves that Stalin could bring up from the depths of the U.S.S.R. The Germans correctly estimated that there were about 150 divisions in the western parts of the U.S.S.R. and reckoned that 50 more might be produced. But the Soviets actually brought up more than 200 fresh divisions by the middle of August, making a total of 360. The consequence was that, though the Germans succeeded in shattering the original Soviet armies by superior technique, they then found their path blocked by fresh ones. The effects of the miscalculations were increased because much of August was wasted while Hitler and his advisers were having long arguments as to what course they should follow after their initial victories. Another factor in the Germans’ calculations was purely political, though no less mistaken; they believed that within three to six months of their invasion, the Soviet regime would collapse from lack of domestic support.

German forces made significant advances early on, and by mid-July the Germans were only 200 miles from Moscow. Arguments within the German high command, however, undermined the German surge, and the invasion began to go awry by August. As winter approached German troops became bogged down, including at the outskirts of Leningrad. Still, German prospects for felling the Soviet Union looked bright for a period (and to many of the generals, who figured the Soviets were near defeat); for example, in October German troops captured more than 600,000 Soviet troops after a great encirclement at Vyazma. That battle, however, tired German troops, and well-clad Soviet reinforcements helped to thwart further German advances. A move on Moscow in December was resisted, and in the winter of 1941-42 the elements took their toll on the German troops, many of whom suffered frostbite because they were not equipped with adequate winter clothing.

Although the Soviet Union’s Red Army suffered greater losses during the campaign, the inability of German forces to defeat the Soviet Union marked a significant setback for the German military effort and perhaps was the beginning of the end of Hitler’s so-called Thousand Year Reich.