Despite being sworn ideological enemies, Nazi Germany and the Communist Soviet Union put aside their vast differences to sign a nonaggression pact in August 1939. Thus assured that the USSR wouldn’t intervene, Adolf Hitler kicked off World War II just days later by invading Poland. Other invasions followed as Hitler and Stalin carved up Europe between them, with the later occupying Estonia, Latvia, Lithuania and large chunks of Poland, Romania and Finland. The Soviets even entered into negotiations to become a fourth Axis power alongside Germany, Italy and Japan.

Hitler, however, considered ethnic Russians to be an inferior “mass of born slaves who feel the need of a master,” and he dreamed of clearing out much of the Soviet Union so that German settlers could procure “living space.” Before the nonaggression pact was even a year old, he began plotting a surprise assault against the USSR, later dubbing it “Operation Barbarossa” after a medieval German emperor. U.S. President Franklin D. Roosevelt and British Prime Minister Winston Churchill both tried to forewarn Stalin, with Roosevelt telling him “it was as certain as that the night followed the day that as soon as Hitler had conquered France he would turn on Russia.” But the Soviet dictator distrusted their intentions and stubbornly clung to his belief that the Germans wouldn’t fight on two fronts (which had doomed them during World War I).

Operation Barbarossa tank. (Credit: Galerie Bilderwelt/Getty Images)

Stalin also ignored his own spies, who, from such locations as Germany, Japan, Romania and Switzerland, reported with increasing frequency that the Nazis were about to strike. In early 1941, for example, an undercover source in Berlin asserted that “war with Russia has definitely been decided on for this year,” whereas in Bucharest a German commander was quoted describing the upcoming clash as “something that goes without saying.” Border guards heard much the same from captured enemy saboteurs, and railroad workers observed huge numbers of Nazi soldiers moving east. Though not every report proved reliable, Soviet intelligence purportedly named the exact, or almost exact, date of the invasion no fewer than 47 times in the 10 days before “Operation Barbarossa” went into effect. The Soviets even recorded wiretaps of Germans discussing Hitler’s plans, including one officer who declared “they haven’t even noticed that we are preparing for war,” and another who said “the Russians, of course, will be taken unaware.”

By mid-June 1941, German border sentries had stopped saluting their Soviet counterparts, German merchant ships had disappeared from Soviet ports, and diplomats from both Axis and Allied countries had emptied out of Moscow. Even many ordinary citizens could see the writing on the wall, such as a group of Polish women who on June 15 reportedly approached a frontier river and shouted across in broken Russian that war would break out in a week. In the skies, meanwhile, Nazi warplanes violated Soviet airspace hundreds of times in the months leading up to the conflict and were shown to be photographing airfields, fortifications and other strategic sites.

Yet Stalin remained largely unconcerned. Dismissing the reports of war as British provocations, he continued shipping raw materials to Germany and ordered his men not to fire on German planes that crossed the border. Rather than focusing on defensive preparations, he occupied himself with still another round of military purges, thereby robbing the Red Army of much-needed experience. The Nazi propaganda machine may have contributed to his false sense of security, claiming, among other lies, that rookie Luftwaffe pilots were simply losing their way and that troops were massing in the East solely to avoid British air raids.

When a German soldier defected on June 21 and declared that an attack would come the following morning, Stalin at last awoke from his slumber, ordering border troops to maintain full combat readiness and to camouflage airfields. Even then, however, he maintained hope that an all-out war with Germany could be prevented. Keeping the Red Army in a partially demobilized state, he directed it “not to yield to any provocation” so as to prevent “big complications.”

Alas, the Germans struck with fury at dawn, bombing previously reconnoitered targets along a 2,000-mile front from the Baltic Sea to the Black Sea and easily smashing Soviet resistance. “Tactical surprise of the enemy has apparently been achieved along the entire line,” a Nazi general wrote. Within weeks, the invading force had pressed more than 400 miles into the USSR and had killed or captured hundreds of thousands of Red Army soldiers, along with innumerable Jews and other perceived civilian foes. The tide of war finally started to turn with the arrival of winter, when the advance stalled on the outskirts of Moscow. Nevertheless, not until 1944 did the Soviets reach the pre-attack line of June 21, 1941, by which point several million people had perished in some of the bloodiest and atrocity-filled fighting ever seen.