Wth the Soviet Union, almost all went historically for me and others until Summer 1941, when Germany firstly never bothered to attack Yugoslavia, (they invaded Greece by moving divisions through Bulgaria) and Barbarrossa never happened. Instead I saw that Germany after defeating France switched to mainly building a navy, and had garrissoned 90% of their divisions along the French coast as if they were expecting an early D-Day in 1941.
I was able to overrun them very quickly. In the first month or two I occupied most of Poland and East Prussia, I also knocked out of the war Finland, Romania and Bulgaria very quickly. But then Germany brought all they had and put my advance to a halt, even driving my forces back in some parts of Poland and Romania.
I had numerical superiority, with at least 35 divisions in each province, in some areas I concentrated up to 80 divisions, while the Axis never had more then 25 divisions in each frontline province, sometimes only 15 divisions, a 1/3 of which were often non-German.
Also, technology levels and uprades were about the same, although in by the of 1945 all my divisions had been fully modernised, while Germans had only partly done so.
I noticed one very frustrating change when it comes to offensives, typically I would attack a province held by 20 Axis "international" divisions from 3 sides with a total of at least 90 divisions, the battle would drag on but still be in my favour with all their units getting close to 0 organistion, when SUDDENLY ONE mere infantry division would arrive from another province, and the battle would drag on with just this 1 division, while all other 20 are too desorganised for combat and no longer play a role in the battle.
It would take at least another 2 days or so to defeat this 1 annoying regular infantry division, by which time another 10-15 divisions would arrive to reinforce them and the battle would go nowhere, or even possibly turn against me as they could launch a counter attack. Historically, it did sometimes occur that just 1 brave division could hold off a 1 million men army with tanks and artillery, but such cases were still very rare, whilst the Wehrmacht had done it to me all the time.
Then Yugoslavia joined the Axis which streched the front further South, I first thought this would play to my advantage as I had many divisions stationed in Romania and Bulgaria and would easily defeat Yugoslavia, that is until Germans sent 25 divisions to Yugoslavia which stalled my advance there too.
I have heard that you can in such stalemate situations open a gap in the front and let them pour many units into this gap, and then close it with a pincer movement to encircle and then destroy them, but I feel this a bit unrealistic, because in reality a qulified General would see that its just an obvious trap and try to use it to his own advantage, not send his army to fall right into the same trap everytime.
This stalemate went on until 1946
