I'll admit it does seem kind of strange that the soviets, with their enormous and triumphant armies would not succeed as much as the original author of this thread desired, but, on the flip side, they'd presumably account better of themselves than the Brits might achieve with
Unthinkable.
The primary problems that various people have posted is that:
1) By 1945, Soviet armies, though larger in numbers suffered from a high casualty rate, which manifested in units being
understrength.
2)
Logistics - much of the Soviet war machine was hobbled by a lack of expanded infrastructure to support their armed forces, and a large portion of their current logistical capability came from Lend-Lease.
(Did the soviets in circa 1945 have the manufacturing capacity to replace destroyed vehicles without diverting overly from tank/apc production if the wallies were no longer supplying them?)
3) Technology - Let's at least agree that certain parts of the allied armies were different enough from each other in that the soviets possessed some advantages over the wallies, but neither side did not retain a monopoly on air, land or sea, with the exception of the Navy, where unless Stalin had hidden nuclear-powered Typhoons from the future in the centre of the earth, was unlikely to match them at best. Yes the IS series was impressive in it's function. Yes, IL-2's were powerful. No, they were not operated by Jesus, Allah, Inari, or your Deity-of-choice, and were therefore not wonder weapons compared to the wallies war inventory. Similarly, the wallies would have faced a numerically superior foe, and for all that they could field and supply a tactically brilliant counter-force that was beginning to focus on combined operations (thanks to a superior air force) to any soviet attempt to subjugate Europe, in order to receive supplies and reinforcements, they'd have to weather the soviet onslaught first.
4)
Suprise!!! - Your premise that the soviets expand and push back the wallies, presumably outright obliterating them as a capable fighting force, is inconsistent. At first, it was a continued expansion. Then as more people posted the impossibility of that occurring, 6 Months. Eventually, I think it evened out at a year and half, and then,
then a surprise attack that....pushes into a Japan that didn't surrender to the US (and a US that didn't/couldn't take northern Japan), takes much of the Middle East, chunks of China, and invades Scandinavia all at once. And surprises the wallies in Germany, who were thoroughly convinced that the USSR was a benevolent country that was building back up the Eastern countries it had overrun before gifting them with the freedom to choose the government of their choice, and totally didn't notice any build-up of forces with their inferior air force, that had been stored out in the open, on tarmacs.
And all around the world the magical happy fairy dust that all the Russian children that were good and had prayed to Grand-pap Stalin floated around the world and put all those naughty fascist/nationalist/democrat soldiers to sleep, just in time for his Red Army to catch them unawares.....
Seriously, you'd have a better POD if, just before the armistice is signed between the soviets and the allies, Stalin seeing that Russia is in danger of collapse, panics, and zombifies everyone in Russia into intelligent zombies, and there's an invasion due to a lack of .....
Brains.
Lots of inane violence, lots of zombies nomming everyone, it gets around part of the supply problem, and you get the third world war you were looking for.