WWIII sometime in the early 1950s, either Korea escalates as happens in that new HT novel or Stalin strikes West as he seriously considered.
Soviets win the War, conquering most of Western Europe. The US adopts a somewhat China-centric policy and so non Maoist forces actually control a good chunk of the country.
The British and the Americans are exhausted and bankrupt, and the end of the war triggers an economic depression. A series of scandals, bad policies etc. ultimately triggers a revolution which spreads elsewhere in Anglosaxony eventually resulting in Ingsoc achieving control of North America and the British isles and then crushing Latin America, the South Africans etc. with overwhelming force.
The Soviets meanwhile undergo a de-Stalinfication in some respects, (perhaps post-Stalinism is a better word) and reform into the Eurasian Union, they are too busy consolidating control of their conquests to do much else.
Eastern Asia is eventually united a bit later. By the mid 1960s all of the world has fallen to these three powers, though one suspects their grip on most places is not as tight as it is in urban England.
The problem is figuring out just how a revolution could happen in the United States given a post 1945 POD, losing WWIII would be humiliating, but I'm not sure it would cause such a collapse.