A conventional war with the Sovs could get you a million casualties if it happened in the 40s or 50s.
But this is like the recipe for rabbit stew--first you have to figure out how to get a conventional war with the Sovs.
Bonus points if it also involves US troops fighting in China, that should tick up the casualty figures nicely.
Or WI you have an WWII equivalent but against a Soviet Union that dominates most of the Continent.
Say the Brits agree to a white peace, the Germans turn their attention to the USSR, Lend Lease still happens, and the Germans get their arses kicked. But when Germany is collapsing and Soviet troops are rolling into Germany/Austria, Stalin is drunk on the heady wine of victory and makes an uncharacteristically incautious address about the unstoppable revolution and Soviet triumph, including menaces to Italy, France, and Spain and implied threats to imperialist powers that might attempt to block the armed might of the proletariat from throwing off their fascist oppressors in these countries. Whether Stalin would have followed through on any of this is highly doubtful, but he dies soon afterwards (probably of natural causes, but better yet if there's a good case to be made that its some western national that somehow manages to assassinate him). And in the power vacuum, the Soviets do follow through, partly because of the heady wine of victory, partly because factions are competing to see who can be more true to Stalin's victory, and partly because the speech provides a convenient platform for the survivors to rally around instead of having to negotiate a consensus political platform in the absence of any obvious successor to Stalin. Powerful Red Army elements support the program too because (1) it keeps them in the center of affairs and (2) its obvious that the USSR will best be protected if there aren't capitalist staging grounds on the mainland with which to threaten the revolution (in other words, despite some wild talk about worldwide revolution, the real goal here is securing continental Europe, though folks abroad don't necessarily know that). Additionally, communists in one of the threatened countries--Italy, Spain, or France--make a revolutionary attempt that has some initial success, and the soviet leadership feels like it needs to support it. Result: before the Soviets have even finished mopping up Germany and Austria and before they are logistically able to launch a serious attack, the USSR declares war/launches a few attacks on these countries.
It helps a lot if we assume that at least one of France or Italy, preferably France, has thrown out their fascist government. Lets say its France, presumably with British help, and that the communists were brought in as part of a coalition government but tried for a premature coup after Stalin's victory speech.
Lets also say that extremist communists in Britain and America engage in attacks and provocations soon after the Stalin speech, carried away on a surge of enthusiasm that the worldwide revolution has finally come.
Now here's where it gets tricky. I don't think FDR, who had a big blind spot where communists were concerned, would be very likely to be willing to fight to defend France against communist invaders. At the same time, I think you need the US to have a military build-up and a discrediting of isolationism, which means you probably need a US war with somebody, probably Japan. But if that happens, FDR probably retains office as in OTL. However, by the time the USSR is on the point of beating Germany, FDR could have plausibly died. We could even have him die earlier than OTL. Lets say he does die in '43 or '44, there's a conservative reaction as in OTL, and coincidentally there is also an earlier discovery of some of the Communist penetration of the organs of American government, which had a notoriously lax approach to security (perhaps, also as OTL, the reaction is just a little ginned up for political purposes).
Up shot is that both the US and the UK commit themselves to the security of France and end up in a what they think is an existential war with the USSR.
Nukes haven't been invented yet and though they are being crash-programmed, they are still at least two years off. The Russians have millions of men under arms, and the Anglo-American high command is convinced that the French foothold on the continent must be held at all costs, because once it is lost it cannot be regained (the earlier experience with the Nazis taking France and Britain subsequently dropping out of the war will only seem to reinforce this notion). France becomes the scene of a desperate defensive struggle. Millions die. Mothers are childless and wives are widows.
