The problem is given the way game politics work there's just no front on the west, and the possible landing sites are Koningsberg, which I rule out because it's exactly one province wide and so is supremely defensible for the Soviet Union; and the vicinity of Leningrad, which is almost as bad, and which I saw the Axis try amphibious assaults upon repeatedly and fail every time. The Baltic Republics and Poland are not part of the Soviet Union or under its domination, so there's no invading through there, and the Black Sea is inaccessible because Turkey will not allow passage of their --- or my --- ships through the Bosporus.
What about the White Sea? If I gather landing forces at Scapa Flow, then it's just at the limits of how far naval transport can bring an amphibious assault force, provided there's no opposition. Since the Soviet Navy consists of several fiercely painted rowboats, I could land the forces who cut through Germany and Italy in seven months there pretty near unopposed. There's only one suitable landing spot --- Archangelsk --- which is not ideal, but, assuming that I can land unopposed and some gentle probing with a destroyer fleet indicates they're not heavily fortified there, there are several provinces which could be swiftly occupied after that landing. (Murmansk offers another landing site, but it's farther north and isn't adjacent to quite so many other provinces.) So I could have a respectable beachhead with several naval and air bases soon enough.
From there, though, Moscow would be a relatively short plunge due south. Or a drive west to take Leningrad from behind would let me take a relatively warmer-ish sometimes-water port. I'd have to supply my forces through the White Sea, at least unless I do take Leningrad, but using the Soviet air bases at Archangelsk, Murmansk, and possibly Leningrad would leave my forces under air force protection all the way to Moscow, which is itself another potential nexus of air-covered advance and lead me to Kiev and down to the Black Sea, severing the Siberian forces from all supply. Of course, they could counterattack and isolate my forces from supply, but Air Transport can supply troops via airlift. If it worked it could make conquering the Soviet Union a wonderful, Liddell-Hartian triumph.
If it could work. Right this minute, of course, it's January 1948 in the game and I'm kind of light on alpine forces. The supply lines would be by construction long and slender. While a lot of the Soviet Army is in eastern Asia there's all those tanks I didn't want to fight outside Leningrad who'd be rushing to fight me in the suburbs of Moscow. In Asia I'm getting reinforcements from the Chinese nations, Korea, and Japan; here, I'd be getting just my own reinforcements shipped in from Boston. Troop supply via airlift is a completely untested technique in this timeline. And even if I did take this longitudinal swipe it would not compel a Soviet surrender: they would still have most of their army and many, many men to replace loses. And did I mention it was winter? And yet ... and yet ...
I feel tempted into something either brilliant or epochally stupid.
Trivia: The British landed troops at Archangel and Murmansk in the spring of 1918, in support of the White Russians; the Japanese invaded Vladivostok similarly, and by the end of the year about 180,000 foreign troops were on Russian soil. Source: Paris 1919: Six Months That Changed The World, Margaret MacMillan.
Currently Reading: Out Of Their Minds, Clifford Simak. This is the one where the older creatures of human imagination are upset that it's now clogged up with characters like Snuffy Smith or Charlie Brown. I believe james_nicoll pondered what sort of rage they're in these days now that Garfield has made the set. I'm not sure whether the resolution is a cheat or not, though. (I think the dynamic where the older folks assume they're better than the newer is a somewhat standard-issue one, although I would agree that, like, Loki is more inherently interesting than anyone on Two And A Half Men.)