I recall a famous cartoon from the 1960's, Russian no less. The image is of a flatbed traincar with one enormous nail riding on it. The caption says "The Program has succeeded." or something of the kind. This project was to build 50 tons of nails, which it did inthe form of one 50 ton nail. My Eastern European friends find this hilarious. Their humor is as obscure as the Brits sometimes.
In any event, it illustrates the problem. The USSR was never a true world power in the sense that the US is. Take away the nukes and they become the biggest power in Europe and nothing more. As they are today. The world would have to be VERY different for the USSR to be the dominant world power.
Here's one scenario. The world is composeed of plates floating on magma. The edges of the plates either push or pull on the plate next door. One of the really big "pull" sites is in the Pruerto Rican trench off, ta da, Puerto Rico. Suppose you drop a really BIG bomb to the bottom? In theory you might shift the whole pair of plates, the Atlantic and the North American. Think a grade 10 or 11 earthquake with dozens of grade 8-9 aftershocks and grade 6 and 7 quakes by the thousand for months afterward. All of this concentrated on the American Atlantic seaboard. Tidal waves half a mile tall. Florida gone. Salt water in Atlanta. Sympathetic destruction all along the Pacific coast and the European coast. The country most immune is the USSR, since its major population centers are far inland.
What then.