What would the immediate and long term effects have been if the US had decided to embrace isolationism (at least temporarily) after WW2? Does this greatly help the USSR?
And in the long term, would control over Germany and a preponderance of power in Europe have saved the USSR?Yes. It is a massive boost to the Soviet Union. All the borderline countries, like Italy, will vote in Communism out of frustration at corruption without the West actively acting against their factions.
Without a strong, coherent, military presence in Germany; keep an eye on the Fulda Gap because Soviet armor would almost assuredly come flowing through sooner or later.
And in the long term, would control over Germany and a preponderance of power in Europe have saved the USSR?
Does this greatly help the USSR?
No. Believe it or not, it would probably make its life more difficult.
But first we have to define isolationism. What would an isolationist US look like? Would it maintain its existing commitments but refuse to take on new ones, a'la the US before 1941? Would it be like China is today, a state which is for the most part politically isolationist (bribing certain foreign politicians notwithstanding) but economically internationalist, or would it be more like Australia: highly interventionist in its region but mostly isolationist everywhere else? Would it be a Swiss-style militarised neutral republic or a Loupist model state which abolished its defence forces and replaced them with an army of strongly-worded-letter-writers?
Except in the Loupist scenario where the USSR is free to invade any country it wishes, things become more and not less difficult for the USSR. How so? There are essentially six major factors that we need to consider in this scenario, and the sort of state the US becomes affects how each of these factors will affect the USSR. They are: Britain, Italy, France, Japan, Germany and China.
Of these six countries, Britain is the only one which we can safely assume will not turn Communist on its own. The CPGB was never a serious electoral force, and even in a world in which communism has more popular appeal worldwide, FPTP makes it highly unlikely that the CPGB could ever win enough seats to form a government barring a Labour split. In all scenarios with the possible exception of the first, Britain would essentially be the leader of the free world in this timeline. IRL Britain played an important role in the early years of the Cold War in stemming the communist tide in Southern Europe and Asia. Britain would still have the means to do this without the Marshall Plan, but the relative burden of doing so would be much, much higher.
The moment of truth for Britain would come in the mid-1950's with the crises in Iran and Egypt: a non-interventionist US would paradoxically increase the chances of Britain retaining control of Suez, but without access to Iranian oil Britain simply could not continue to function as a world power. The best case scenario for Britain would be one in which France remains Gaullist, Britain remains capable of deposing Mossadeq on her own or with French help, and the US remains interventionist enough to at least ply the British government with money and/or favourable trade deals while not caring too much about the Empire or her actions in the Middle East. This would be the FU wet dream scenario, in which an Anglo-French Entente takes up the mantle of world policeman and fights a cold war against the USSR. Instead of the US and USSR vying for influence in independent third world countries, you would have the USSR funding independence movements in Anglo-French colonies. It is conceivable that either side could win this cold war, but it would take a lot longer to resolve than in OTL given the relative weakness of the Entente relative to the US.
A much more likely scenario would be one in which the British (or Anglo-French) economy collapses under the weight of its growing defence obligations, essentially mirroring the fate of the USSR in OTL. Of course this would happen to Britain much, much earlier than it happened to Russia: probably around the mid-1960's, though it's conceivable that Britain could have continued well into the 70's if she retained control of Iranian oil and/or was in bed with an American sugar daddy. In the aftermath Britain would cease to be a combatant in the Cold War, but nor would she likely join the Soviet bloc outright. Instead, she would probably assume a role reminiscent of Russia during the Yeltsin Era: not a part of the Soviet bloc, but not openly antagonistic towards it either.
France could potentially go down the same route, depending on how close ties remained between it and Britain. Unlike Britain though, France could certainly turn Communist unilaterally; though I would stress that this was by no means guaranteed and even if it did France wouldn't necessarily automatically choose to join the Soviet bloc. Again, this is mostly depending on how the US behaves towards France in this scenario. For instance, an Asia-focused US could find common cause with France over Indochina, or Indochina could be a potential cause of friction if the US chose to sympathise with the rebels.
Italy would almost certainly go Communist regardless of what form the US took. IRL the 1948 election was a proxy war between the cold war powers, with the CIA bankrolling the Christian Democrats and the KGB the Communists. Even in the most interventionist scenario where the US retains an occupying garrison in post-war Italy, it's hard to envision a scenario in which the Garibaldi Front fails to win the elections without the significant CIA intervention we saw in OTL -- it's entirely possible, indeed likely, that the Catholic Church and the mafia would still aid the anti-Communist parties, but they would be considerably less effective without CIA help. Even if the Garibaldi Front lost the 1948 elections, the lack of CIA aid combined with the deteriorating economic situation would almost guarantee a PCI electoral victory at some point during the 50's.
The late 1960's is where things get interesting for the USSR: Duclos, Thorez and Togliatti were all Stalinists in the broadest sense of the term, and would have been receptive to Moscow and the party line. Thus, France and Italy would be assets to the USSR in the short term, and even if they remained independent of the WP relations between them and Moscow would have remained cordial. The problem is that their successors were not, and that as highly charismatic and strong-willed individuals with close ties to the outgoing leader, both Marchais and Berlinguer would likely still have ascended to the leadership of their respective communist parties in this timeline. Hungary, the Sino-Soviet Split and the Prague Spring -- or some similar events -- would have occurred in some form regardless of what the US did, and the conditions for the advent of the New Left would still broadly be in place. As Italy and France would have joined the WP willingly in all but the last scenario, they would retain enough independence from the USSR to act independently if they so wished; they wouldn't be puppets and thus couldn't be cocerced in quite the same manner as a Czechoslovakia or a Hungary.
This would all cause some massive headaches for the USSR. Instead of a protest against Gaullism, the '68 protests would be directed against Stalinism. Instead of impotently denouncing the expulsion of the Chinese from afar, Berlinguer's stance would carry real political weight coming as it did from the second or third most influential Communist power. It would provide a catalyst for a much larger Sino-Soviet split with France and Italy supporting the Chinese, and/or for the foundation of a third Eurocom bloc between states who independently chose communism and those Russian puppets who had it thrust upon them. This bloc could in turn be pragmatically supported by the Americans and maybe the British much as the US backed Mao and Tito in OTL -- if the Empire collapses and Britain turns socialist, they could even formally ally themselves with the Eurocoms -- and potentially go on to "win" the Cold War outright.
So with all due respect to LBJ, it would be far better for Moscow to have them outside the tent pissing in than to have them inside the tent pissing out.
Japan meanwhile is Italy or Yugoslavia on steroids. Obviously if the US retains interests in Asia it won't turn Commie, but assuming they don't a socialist Japan has the potential to really screw things up for the Soviets. Japan's economy recovered quickly after the war and IRL benefited from a great deal of dirigisme, so unless the US is prepared to wage economic warfare against it it's entirely plausible that the Japanese economy would still take off under socialism. Even if you exclude the bounce it received from US investment during the Korean war, Japan could still become the economic and scientific powerhouse it was in OTL, it just might take a few extra years to achieve.
The JCP were a lot weaker than the PCF or PCI electorally, but a lot of this was due to pervasive US influence in Japan -- MacArthur effectively did everything to curtail the party's electoral prospects outside of banning it outright; a leftist victory is therefore certainly plausible assuming the US does not retain its iron grip on Japan. Assuming that the JCP or a popular front do win power, Japan would be unlikely to join the Soviet bloc. Miyamoto was a Eurocom long before Eurocommunism was a thing: he advocated Dubkecism a full decade before Dubkec himself, denounced both the Soviets and the Chinese, criticized both NATO and the WP and repudiated both Stalinism and Leninism way back in the late 50's. In short, he was the closest thing the world got to an electorally successful Left Oppositionist, and would be a major thorn in the Soviet's side.
The only way for Japan to become a net asset for the USSR would be to either rewrite the end of WWII as a Russian rather than US occupation, or to have Miyamoto meet the same fate as Dubkec. The latter would be difficult to achieve without violence, but given the JCP's pacifistic stance in OTL violence wouldn't be difficult to achieve. The exception of course would be if Miyamoto foresaw a Soviet invasion by choosing to pragmatically rearm, much as the Lib Dems did IRL. Either way, invading or potentially nuking the world's most successful non-European Communist state would not be a good PR move for the Soviets.
Germany is far too difficult a variable for me to accurately predict. Even if the US relinquishes its garrison, West Germany could still exist in this timeline if Britain and France chose to retain theirs (which I think quite likely, even in a state of severe economic hardship.) Russia could easily overrun such a garrison, but that applies in OTL as well, and in this TL Britain and France would still have nukes. Without the US and the Marshall Plan, I would expect the British to be much less keen to foot the bill for Germany's defence and thus be more tolerant of German rearmament.
Even if the Soviets united Germany under communism, I can't say for certain whether Germany would prove a loyal client state or not. If the same people who ran the DDR in OTL ran the new Germany, I think it likely, but it's entirely possible that an influx of Western Leftists combined with the absence of the West as a galvanising rival force would have resulted in a more independent, if not outright Eurocom, DDR. On balance though a unified Germany would have probably been a net asset to Moscow, and a useful counter-balance to the Paris-Rome-Tokyo axis.
Unless the reason for US isolationism in Europe is due to massive interventionism in the Orient, China would likely proceed as it did historically. The difference in this timeline would be that it would have a lot more potential friends to play with. Thus, even if the US decides to play no role in China's affairs at all, China could still find herself making friends with Western (Commie or otherwise) powers in order to counter the threat posed by the USSR. I don't forsee US isolationism altering Sino-Soviet relations at all.
tl;dr US isolationism might be good for communism, but unless the USSR decides to go full Trotsky and the US full Loupie, I don't foresee it being better for the USSR. Communism was screwed the second fate chose Russia to be its standard-bearer.
And in the long term, would control over Germany and a preponderance of power in Europe have saved the USSR?
It would have let the Soviets play hegemon in Western Europe, spend less on their military, and press France and the other continental powers to trade technology and advanced machinery with them in exchange for oil and other resources. A dream come true for the historical USSR of the 1970s and beyond. It's what they dreamed of when they clamored for the USA to leave Europe. Of course it would help them immensely. Would it make the USSR survive indefinitely? Hardly.And in the long term, would control over Germany and a preponderance of power in Europe have saved the USSR?
And in the long term, would control over Germany and a preponderance of power in Europe have saved the USSR?