After Germany had declared war to the Soviet Union during WW2 by invading Ukraine first and by taking over Smolensk despite the attempts by the Red Army to stop them, the Stavka (Russian High Command) after beeing sure that Japan would not get involved in Siberia had recalled all of their Units from Siberia and transfered them to Moscow defense hoping the bulk of the Red Army would slow the Wermacht down long enough to launch a winter counter-attack.
So the bulk of the Red Army was stationned to west of Mozhaisk, forced to pick on German supply lines. The Russian Partisans (mainly civilians that after seeing the horrors the Waffen SS were bringing and had sided with the Red Army) were especially good at this job often supplied directly by the Russian Air Forces and sometimes receiving a Russian staff officer in order to coordinate their moves with the regular Army.
But it seemed as long as Winter wasnt there, nothing could stop the German Armies with their Finns, Italians, Rumanians, Hungarians, and Slovaks Allies, until Winter struck.
Since the OKW had never planned for a winter campaing near Moscow they were ill-equipped and it had a devastating effects on their gears, planes couldnt fly, guns were freezing, tanks were easely destroyed but on the Russian side, they had plenty of what they needed to carry out that kind of operations, they also had Battle Experienced Troops that fought the Imperial Japanese Army during the 1930s with General Zhukov in Command, to make things even worst, the counter-attack had followed the parade in the Red Square with Stalin himself reviewing his troops that had arrived from Siberia with a propangada effort, if they had lost Moscow, it was all over for the Soviet Union itself and the War would also have been lost for the Western Allies (including UK, the Free French and the USA) but everywhere the Germans had invaded, the Germans had taken heavy casualties.
Then starting on December, the Russians launched their Counter-Attack with their Siberian Divisions, and after causing the Germans to retreat until February when it got even worst to the Germans due to the mud on the ground and the heavy gear such as Tanks, Vehicles such as APCs, Trucks, Motor-Guns, Cars, couldnt even move yet again the Russians didnt had that problem, and had introduced new Tanks, new Planes etc and had captured many German Soldiers and made them parade in Moscow heavily guarded by Russian Soldiers, the War was barely starting for the Russians as they were launching full mobilization in the entire Soviet Union and American/British stuff was already had already arrived in Murmansk north of Leningrad (known as St-Petersburg today).