Halifax rather than Churchill becomes leader of Britain and takes Britain out of the war. Because of this, Stalin becomes less mislead about the possibility of a German attack (OTL, he suspected the whole thing to be a British plot). Britain sits out of the war, rebuilding its army and the like in order to face off against Germany at a later time with a continental ally if it can find one as Britain has done for generation.
The German offensive into stalls out early due to better Soviet preparations, and with support now flowing from America and Britain, the Soviets roll back the Germans in about the same time as they did in OTL. Britain launches an invasion of France as the Soviets reach Berlin, but it is pretty clear who now dominates the continent.
Meantime, in the Pacific, the USA has been busy pounding Japan, and in 1945, persuade the Soviets to enter the war, launching Operation August Storm against Japanese possessions in China, and help install a Chinese communist government.
Eurasia is now effectively communist from one end to the other, with only small peripheral states like Britain and whatever rump of France it can liberate before the soviets getting there being capitalist.
What happens next?