I'm going to ignore the "how" of this in favor of talking about such a conflict itself.
First and foremost, there is no overwhelming Soviet manpower advantage and the ratio to the West is close to a 1:1 basis than what I've seen suggested within this thread. The Soviets did have more
divisions, but that is merely a paper advantage given they were managing 2-5,000 men per division at this time as compared to over 10,000 for your regular American unit. By late 1942/early 1943
the Soviets had effectively exhausted their manpower according to reports delivered to Stalin.
A translated summary of it is here:
In Army, Navy, NKVD troops - 10 947 000 () men plus 851 000 in hospitals class of 1925 in process of call-up - 817 000 recruits transferred to industry work - 2 541 000 discharged or on leave for medical reasons - 982 000 nationalities exempt from military service - 250 000 irrevocable causalities - 5 950 000 lost on occupied territory - 5 631 000 + 965 000 (classes of 1924-1925) A remainder (not called up yet) - 3 724 576 (of them about 2.5 million reserved in economy)
GULAG, NKPS troops and other minor military seem to be forgotten
() Breakdown of military: On the front - 6 191 350 men Far East - 1 131 696 internal military districts - 1 932 995 (including 1 422 659 in replacement, training units, and military schools) others - 744 901 (reserves, units in transfer, airborne forces, separate air force and air defense elements) Total Red Army - 10 000 942 About 946 000 in Navy and NKVD (calculated from the difference)
So in general, the Soviets had about ~3.7 Million left to call up, of whom 2.5 Million were needed to keep the economy going and most of the rest were Central Asian natives, whom were not exactly ideal to call up due to reliability issues. Even if they do call them up, that's just an additional 1.2 Million, which for reference is essentially the casualties incurred at Kursk and Smolensk in 1943 alone. What ended up saving their bacon was battlefield success,
in that they liberated territory that provided the manpower to keep going. Still, even by 1944 they were on their last legs:
Bagration, 1944, Osprey Campaign Series -
"Soviet rifle divisions were generally smaller than their German counterparts, averaging 2500-4000 troops. At the time of Operation Bagration a concerted effort was made to bring these units up to an average of 6000 troops. No serious effort was made to bring them up to their nominal TOE strength of 9600 troops."
Red Army Handbook, 1939-1945, by Steven J. Zaloga -
"By this time, however, it was becoming apparent that the Soviet force structure of 500-plus Divisions simply exceeded their capacity to support it. [..] in mid-1944 more drastic action was necessary. Either some of the rifle divisions would have to be demobilized and their personnel used to fill out other units, or divisional strengths far below envisioned norms would have to be accepted. The Stavka opted for the latter alternative."
Soviet Military Doctrine from Lenin to Gorbachev, 1915-1991, by Willard C. Frank -
"Soviet sources reflect manpower deficiencies by emphasizing the low strength of rifle units and the draconian measures used to enlist soldiers in liberated regions. By 1945 Soviet rifle divisions were often under strength, with only 3,500 to 5,000 men each."
Next, we move to the nuclear issue. Serial production was already underway by the end of 1945 and for the first half of that year the masses of B-24s and B-17s the U.S. had used to break Nazi Germany were still present. To understand why this is important, consider the following:
Take out Warsaw, Lublin and Lwow, and the entire logistics net of the RKKA West of the Vistula immediately collapses as they've just lost their rail connections to the USSR proper. The Soviets could not recover from this due the following:
92.7% of all Railway rails were Lend Lease sourced.
81.6% of all Locomotives were Lend Lease sourced.
80.7% of all Railcars were Lend Lease sourced
Source for all of these.
59% of all AV Gas also came from the West, which is critical as
Air Power and Maneuver Warfare by Martin van Creveld states that 87% of German counterattacks against Soviet exploitation forces happened outside the range of all fire support except for the Soviet air force. With such a steep reduction in AV Gas, the VVS will be unable to play this vital role and most Soviet attacks will collapse and be destroyed in the face of Anglo-American counter-attacks. This is already no much of an issue, given overwhelming Western air advantage.
Finally, the entire Soviet economy was threatening to fall apart at this time, according to
Beginnings of the Cold War Arms Race: The Truman Administration and the U.S. Arms Build-Up by Raymond P. Ojserkis:
"There was evidence indicating that the Soviet economy was weak. Even the Soviet government's published statistics, which were thought to be generally exaggerated, revealed an economy far behind the west. Soviet diplomatic actions in the immediate post-war period, whether in the form of attempts to gain more favourable conditions for Lend-Lease payments, Sovietlobbying for a large German reparations payment, Soviet demands to gain Austrian oil, or the transportation of basic infrastructure from conquered eastern Europe to the Soviet Union all indicated economic deficiencies. General Walter Bedell Smith, a future head of the Central Intelligence Agency, estimated that it would be another 10 to 15 years before the Soviets had recovered from the last war. The CIA's Office of Research and Estimates (ORE) tried to appraise the Soviet Union in terms of war potential, looking at the industrial strength, technology, and possible bottlenecks to increased production. The ORE concluded that Soviet economic weaknesses gravely limited the ability of Moscow to fight a prolonged war with the North Atlantic Treaty nations."
"In particular, American analysts felt that the Soviet petroleum industry would find it difficult to produce enough high octane fuel, the Soviet machine tool industry did not produce enough spare parts, there was insufficient rolling stock to handle war time needs in the USSR, and the Soviets had perennial shortages of certain non-ferrous metals and certain types of finished steel. Complicating these problems, and, to an extent, causing them, were the Soviet deficiencies in properly trained technological personnel and managers."
Long story short, the Soviets collapse within six months with minimal Western losses.