Stalin dies 1937. Malenkov tries to assume power, but is brutally put down by Beria, who is in turn brutally put down by the Red Army. Marshall Tuchachevsky takes over as supreme Soviet Leader.
All is not well in the USSR however. Plenty of hard-core Communists remain, and Tuckachevsky's mild attempts at reform at met with resistance, which leads to hard reprisals by the Red Army.
Things are choatic enough that Tuchachevsky still inks that deal with Hitler. The Red Army still trashes the Japanese in Manchuria. There is no Winter War - Tuchachevsky's feelers of friendship to the west are still largely rebuffed, but there is more interest.
Handwaving away, WWII goes as in OTL. This is both good and bad for Russia; Tuchachevsky - unlike Stalin - has no problem believing the Germans are about to attack, and there is sanity in the Red Army's deployment in June 1941. No Purges mean a better officer corps. Unfortunantly, the lessons of the Winter War are unlearned, and the Germans slice through the Red Army as in OTL. A much better fighting retreat is managed, and the Red Army falls back in good order.
Unforunantly, at that point (October), the hard-liners stage a coup. Trotsky returns from Mexico, and leads a coup that kills Tuchachevsky, most of Stavka, and the policial leadership. Zhukov leads a counter-coup and kills Trotsky, but the damage is done. The Nazis take advantage of the disorders to seize Leningrad and Moscow, and the Red Army falls back to Gorki, Saratov and Stalingrad.
The rest of the war is more brutal. The Red Army uis greatly weakened, but better led. Zhukov is desperate, and with most hard-line Communists killed by the two coups, willing to embrace sensitble market reforms in return from aid from the WAllies. The Nazis advance deeper in 1942, but reach the end of their logistical tether short of Gorki.
The IJN srill commits national suicide at Pearl Harbor in Dec of 1941. Again with the Hand-Waving, the PTO goes about as in OTL. The US and UK fight back, and the war ends in mid 1946, with Hiroshima, Dresden, Berlin and Brelau as atomic ruins.
The Soviet Union is however devastated by the extra year and depth of Nazi occupation. The Red Army and US Army met at Warsaw, not the Elbe. The USSR is shattered, and in no position to threaten anyone. Furthermore, the political leadership of the USSR is distinctly un-Communist in many ways - including allowing for private land ownership to boost agriculatural production. There is no Warsaw Pact, nor NATO, and exhausted USSR acquiece to the idea of a sole Superpower - the USA.
The extra year of war also cost the US an extra 250,000 casualties, and made for an even bigger military. The US is weary of war as well, and happy to accept a strong, but only semi-communist USSR.
In China, Mao still wins, especially as the USA diverted more resources to defeating a tougher Nazi Germany. Maoism spreads throughout Asia to the disintrigrating European colonial empres. Ghandi adopts a non-violent form of Maoism for India, which lasts until his assasination in 1947. With violent, agrarian Maoist sentiment spreading thtough Asia and eventually Latin America, the USA is too busy putting out brush fires to worry about the USSR. With Maoism spreading to the Soviet Turkic Republics, the USSR is too. The USA and USSR stay allied against this tide...
Mike