In December 1940, Hitler announced Operation Barbarossa, a massive German invasion of the Soviet Union. Hitler was dismissive of the Soviets, famously proclaiming, “We have only to kick in the door and the whole rotten structure will come crashing down.”

In Hitler’s mind, the communist nation was populated by “subhuman” racial and ethnic groups like Jews and Slavs. Those racially “inferior” groups would either be killed on the battlefield or imprisoned as slave labor for the Aryan German conquerors, who would colonize the vast and fertile Soviet lands for Lebensraum or “living space.”

“For Hitler, Operation Barbarossa wasn’t just about defeating Communist Russia, but completely exterminating Communist Russia — wiping it off the face of the Earth,” says Jonathan Trigg, a historian and author of “The Battle of Stalingrad Through German Eyes: The Death of the Sixth Army.”

As it happened, the Nazis vastly underestimated their enemy. Hitler and his commanders based their low opinion of the Red Army on Russia’s poor performance in World War I, but a lot had changed in 20 years. Under a brutal, totalitarian system, Stalin and the communists had transformed a weak and inefficient Tsarist Russia into a “military and economic behemoth,” Trigg says.

Operation Barbarossa was launched in June 1941 with the largest German troop mobilization to date, more than 3.5 million Nazi and Axis troops, 3,400 Panzer tanks and 2,700 aircraft. Hitler’s plan was to attack along three fronts simultaneously: Leningrad in the north, Ukraine in the south and the capital Moscow in the center. He predicted that all three would be captured in 10 weeks.

And at first, things went according to plan for the Germans. The Nazis executed ruthless bombing campaigns against Soviet airfields and cities, and the Panzer divisions captured hundreds of thousands of Soviet troops. But despite these early victories, the Germans were unable to secure their targets.

Stalin commanded a Red Army of 5 million men, and he fed a steady stream of soldiers to defend these cities — or die trying. Retreating Soviet soldiers were routinely shot by their own commanders.

By the fall of 1941, torrential rains turned Russia’s dirt roads into impassable quagmires, and then freezing winter temperatures set in, forcing the Germans to halt their invasion until the next summer.

Stung by his failure to take Moscow, Hitler came up with a different strategy for 1942. He believed that Germany’s Achilles’ heel was its lack of domestic oil reserves, which meant the German military was constantly short on fuel. Instead of re-invading Moscow, as everyone expected, Hitler launched Operation Blue, a long march south into the Caucuses to capture the Soviet region’s rich oil fields.

Stalingrad was directly in the path of the Nazi advance south, but the German high command didn’t see its capture as strategically critical.

“In all of Hitler’s meetings at the time with senior commanders, all he kept talking about was ‘Oil. Oil. Oil.’ — That’s what was key,” says Trigg. “Stalingrad was never mentioned.”

In August 1942, the powerful German 6th Army arrived at Stalingrad under the command of Friedrich Paulus.

“Paulus did what the Germans usually did, bombed the city massively in the hopes that the Soviets would run away,” Trigg says.

Paulus ordered a massive aerial bombardment of Stalingrad, a narrow city that ran north and south along the Volga River. The relentless bombings were followed by heavy artillery strikes that reduced large swaths of the city to rubble. Then it was time for the German infantry to move in, led by Panzer tank divisions.

“What Paulus found out was that in an urban environment, where the streets were pitted with craters and covered in rubble, the Panzers were worse than useless,” says Trigg.

The Battle for Stalingrad became a street-by-street, house-by-house fight, and the Soviets were able to force the German tanks down impassable streets and trap the infantry behind them. Exposed, the Nazis were easy targets for Soviet snipers and even makeshift Molotov cocktails dropped from rooftops.

Paulus, who wasn’t a terribly innovative commander, believed the best response was more firepower, says Trigg. More airstrikes and more artillery fire.

“Yes, it caused terrible casualties to the Soviet defenses at Stalingrad, but it made the terrain all the more difficult for the tanks to negotiate,” Trigg says. Meanwhile, the German supply lines were stretched thin and were unable to refuel and rearm the 6th Army for a continuous attack.

As the weeks turned into months, Stalin saw an opportunity to make a statement at Stalingrad, the city bearing his name. In October 1942, Stalin issued Order No. 227: “Ni Shagu Nazad!” (“Not One Step Backward!”). There would be no Soviet retreat or surrender at Stalingrad, even if it cost tens of thousands of Soviet lives, both soldiers and civilians. Those who surrendered faced military tribunal and possible execution.

“‘We will drown the Germans in our blood’ — that was the Soviet way of making war,” Trigg says.

Stalin sent Gen. Vasily Chuikov to command the surviving Soviet forces in Stalingrad, pinned between the advancing German Army and the Volga River. Instead of digging in and sacrificing more and more Soviet soldiers to the German killing machine, Chuikov decided to go big and try something the Soviets had never done before: a pincer movement.

The flanks of the German 6th Army were defended by Axis allies from Romania, Hungary and Italy, who were poorly trained and exhausted.

“In terms of morale, they didn’t really want to be there,” Trigg says.

Chuikov organized his men into two divisions and launched a surprise pincer attack Nov. 19, 1942. The Romanian, Hungarian and Italian troops put up a valiant fight, but were quickly overpowered. And before Paulus could react, nearly a quarter million German troops were fully surrounded and choked off from their supply lines.

Instead of trying to break out immediately, Paulus and the German high command decided to hold their ground and resupply by airlifts. But the airlifts were a disaster and couldn’t get nearly enough food, fuel and ammunition to the encircled German troops. As the forbidding Russian winter set in, German soldiers began dying from hunger and exposure. Meanwhile, the Soviets continued their relentless campaign, no matter the cost.

“Throughout December, the Germans were ground into dust,” Trigg says. “Their ammunition was getting critical, fuel was absolutely dreadful, hunger was really starting to kick in, and the Soviets were just throttling the life out of them.”

The propaganda machines in both Germany and the Soviet Union turned Stalingrad into a “must-win” for both sides, and Hitler refused to give in even as his men starved by the thousands or were carted off to Soviet prisoner of war camps. The last remnants of the once-vaulted German 6th Army finally surrendered in February 1943.

In all, an estimated 500,000 German troops were lost at Stalingrad, including 91,000 prisoners of war. Of those, only 6,000 German prisoners ever made it back home.

Stalingrad is often cited as the military turning point of World War II, after which Germany could never regain its tactical advantage. But Trigg says that the stunning German defeat at Stalingrad was important for another reason.

“It was a turning point in terms of the German people and their mindset,” says Trigg. “Every single newspaper in Germany published death lists of the soldiers killed in Stalingrad and they were endless. Hardly a family in Germany wasn’t personally touched by the defeat — the loss of a son, brother or close friend.”

No amount of Nazi propaganda could deflect from the fact that Hitler’s glorious invasion of the Soviet Union was an all-out failure.