"The Germans were not equipped for winter warfare, and the bitter cold caused severe problems for their guns and equipment. Furthermore, weather conditions grounded the Luftwaffe from conducting any large scale operations." (https://en.wikipedia.org/wiki/Operation_Barbarossa#Phase_two)
There were 60,000 of them. They marched 20 abreast. They wore soiled uniforms and sober expressions. Their parade took hours to complete. Russian soldiers marched on either side of their column, armed with bayonet rifles. In Moscow, spectators spit on them as they passed by. How had the Wehrmacht, an army which goose stepped across Europe in the first part of the war, been forced to surrender?
Spires of Kremlin silhouetted by German Luftwaffe circa 1941 courtesy http://www.gettyimages.ca/detail/news-photo/overall-of-central-moscow-with-antiaircraft-gunners-dotting-news-photo/53368529.
When Hitler commenced Operation Barbarossa, the Wehrmacht waltzed into the Soviet Union and occupied republic after republic: the Baltic states, the Ukraine, Moldavia and Belorussia. They seemed to be racing towards Moscow uninhibited.
By September of 1941, Hitler announced Operation Typhoon, the drive towards Moscow. The Wehrmacht occupied Leningrad where they commenced a two and half year long siege, starving the city into submission. An early battle took 500,000 Soviet prisoners, bringing the total to 3 million. The Soviet Union had a mere 90,000 men and 150 tanks to defend Moscow. The German government announced the imminent capture of Moscow and the collapse of the Soviet Union.
On December 2, 1941, the German troops were within 15 miles of Moscow. They could see the spires of the Kremlin. This is where Mother Nature intervened: the first blizzards had already begun. Dressed in the same military uniforms as the troops in North Africa, the German troops struggled. "The Germans were not equipped for winter warfare, and the bitter cold caused severe problems for their guns and equipment. Furthermore, weather conditions grounded the Luftwaffe from conducting any large scale operations."
In the meantime, newly created Soviet units, numbering 500,000 men, launched a counter attack on December 5. By the end of 1941, the Germans had lost the Battle of Moscow. Casualties numbered 830,000, many of whom were POWs.
In July 1942, Hitler commenced Case Blue, an attempt to capture Russia's oilfields. Once again, the German Army conquered huge areas of the Soviet Union. However, their advance came to a halt with the Battle of Stalingrad in February of 1943.
In June of 1943, Stalin commenced Operation Bagration which ended in August of 1944 with a decisive victory for the Russians. The German advance had finally come to an end. Millions were captured and sent to Soviet labour camps. The surviving soldiers made a hasty retreat out of Russia, their army decimated, their air force non existent, their horses starving.
German POWs march in Moscow after Operation Bagration in Belarus circa July 1944 courtesy http://rarehistoricalphotos.com/german-prisoners-march-moscow-defeat-belarus-operation-bagration-17-july-1944/.