Operation Barbarossa in rare pictures, 1941

Operation Barbarossa (Unternehmen Barbarossa) was the German codename for Nazi Germany's invasion of the Soviet Union during World War II, which began on June 22, 1941.

It was a turning point for the fate of Adolf Hitler's Third Reich, in which the failure of Operation Barbarossa resulted in the final overall defeat of Nazi Germany.

The Eastern Front, which was opened by Operation Barbarossa, would become the largest theater of war in World War II, with some of the biggest and most brutal fighting, horrific loss of life, and pitiable conditions for Russians and Germans alike.

In August 1939, as Europe headed for another world war, Germany and the Soviet Union signed a non-aggression pact. Given the ideological differences between the two countries, the Nazi-Soviet pact came as a complete surprise to the other countries.

This marked the beginning of a period of military cooperation that allowed Hitler to ignore Western diplomatic moves and invade Poland.

Stalin's army then attacked from the east, completing the subjugation and division of the Polish state. For the next year and a half, Germany also benefited financially from this arrangement, with Russia exporting grain and oil in exchange for manufactured goods.

Soviet cooperation allowed Hitler to expand his plans for European domination. Blitzkrieg rolled west in May 1940 and France was conquered in six weeks. But peace with Russia will not last. Hitler always wanted Germany to expand east to gain Lebensraum or 'living space' for its people.

After the fall of France, Hitler ordered plans to be drawn up for an invasion of the Soviet Union. He intended to destroy what he saw as Stalin's 'Jewish Bolshevist' regime and establish Nazi hegemony.

The Soviet Union's conquest and enslavement of racially 'inferior' Slavic populations would be part of a grand scheme of 'Germanization' and economic exploitation would go far beyond the expected military victory.

Despite recent economic and political cooperation, the Soviet Union was considered a natural enemy of Nazi Germany and a major strategic objective.

On 18 December 1940, Hitler issued Führer Directive 21, ordering an invasion of the Soviet Union. The German military plan called for an advance to an imaginary line running from the port of Archangel in northern Russia to the port of Astrakhan on the Caspian Sea – the so-called 'A-A Line'. This would put a large part of the Soviet population and its economic potential under German control.

After a five-week delay of the completion of operations in Greece and Yugoslavia, Operation Barbarossa – named after the all-conquering medieval Holy Roman Emperor Frederick I – was launched on 22 June 1941. More than three and a half million German and other Axis troops attacked the 1,800-mile front.

A total of 148 divisions - 80 percent of the German army - were committed to the enterprise. Seventeen Panzer Divisions, formed into four Panzer Groups, formed the vanguard with 3,400 tanks. They were supported by 2,700 aircraft of the Luftwaffe. It was the largest assault force ever.

The German army was divided into three army groups, each with a specific purpose. Army Group North was to capture Leningrad through the Baltic states of Latvia, Lithuania and Estonia.

Army Group will attack south of Ukraine towards Kyiv and the Donbass (Donets Basin) industrial area. Between them, Army Group Center was aimed at Minsk, Smolensk, and then Moscow itself. Hitler expected all these to be achieved in about ten weeks.

The Soviet Union had accumulated large forces on its western frontier, but they were under orders not to provoke the Germans. Although distrustful of Hitler, Stalin did not believe he would attack so quickly, despite ominous German construction and a stream of intelligence warnings.

He had about five million men immediately available and a total of 23,000 tanks, but the Red Army was still unprepared when the Germans attacked.

The Germans made a good start, the Panzer groups were rapidly moving towards their objectives and the Russian army was in confusion. They were greatly helped by the Luftwaffe's bombing of Soviet airfields, artillery positions and troop concentrations. The Germans quickly established air superiority.

Army Group Center under Field Marshal Fedor von Bock also made rapid progress. By June 28, Panzer Group 2, led by General Heinz Guderian and General Hermann Hoth's Panzer Group 3, had surrounded three Russian armies and held more than 320,000 men in the Bialystok-Minsk pocket.

The two Panzer groups advanced further on the far side of Smolensk on 27 July, adding in another double envelope. Two more Russian armies were trapped and destroyed, and another 300,000 soldiers were taken captive.

Army Group South, under Field Marshal Gerd von Rundstadt, was the farthest to go, and its attack also faced the toughest of Soviet resistance. Most of the Russian armor was on this front. But in early July von Runstedt had crossed the pre-1939 Polish border.

General Ewald von Kleist's Panzer Group 1 was slowed by Soviet flanking attacks as it headed towards Ukraine's capital, Kyiv, and key to the coal-rich Donets Basin.

On 8 August the Germans surrounded two Soviet armies, captured 100,000 men in the pocket of Uman and reached the Dnieper River. The naval port of Odessa on the Black Sea was also besieged.