Operation Barbarossa was the German invasion into the Soviet Union
They aimed to capture oil fields and to take Russia out of the war
During this time the Germans were using mass amounts of resources in the arms race against Russia and the Western Front with the British and Americans. The German High Command realized their country's shortage of supply and had plans to invade Russia from even the start of the war.
The invasion went very easily as the Germans with their armored division plowed through the Russians
Army Group Centre objective was to take Moscow which in theory the Germans believed that it would at the same time take Russia out of the war.
Army Group South and North would take oil fields and resource areas to supply the advancing German war effort.
However, even with the precedent speed that the Germans were advancing towards Moscow it was taking longer than expected, and it was a growing concern that the German army would not be able to reach Moscow until winter came.
Hitler and the High Command ignored this problem and told them to keep going. It may seem like they made a foolish mistake but it does have reasoning behind it. As Germany is gulping up fuel for its armor divisions and resources for its army far from Berlin. They needed the Russian oil fields to keep the advance going. So Hitler took a risk by telling the Operation to continue.
Once they reached Moscow the Germans heavily outnumbered the Russians, but then winter hit.
The Germans were bogged down and the attack halted as they were not prepared for the heavy Russian winter. This was also helped in the scorched earth tactic which they burned many towns and cities near the German frontline that when they do break through the Russian lines that they would be nothing to take from the towns and cities as they were all burned to the ground. Making malnutrition and hypothermia a constant threat as supply lines were overstretched any severely limited.
Then began the Russian winter counteroffensive
The Russians slowly pecked away at the defenseless German army as the Germans have never been trained to survive in such conditions
Russian winter are recorded to reach temperatures as low as -44 Farenheit
They experienced mass frostbite and many did not survive
The German commanders Hans Schmidt and Wolfram Freiherr von Richthofen ordered a retreat against what High Command had told them
Operation Barbarossa had ended in a complete failure with the German army in slow retreat and the Luftwaffe not there to support the army as the frozen temperatures were creating technical difficulties in the plane.
This was followed by Operation Bagration with the destruction of Army Group Centre and most of the territory back in Soviet Control. The Germans would have to face a defensive strategy until the end of the war.