Hi guys, I've recently read a book on the matter. It is called 'Operation Unthinkable', by Jonathan Walker, and it deals with real plans made by Churchill just before the fall of Germany in 1945. It seems Churchill was really worried about the expansion of the USSR, and even though they were still allies, he ordered his staff to study the possibilities of a successful invasion of the USSR.
Churchill needed the USA to support the attack, but Americans were still engaged with Japan, and nuclear bombs were under secret development. In fact, Americans were looking forward to the end of the war with Germany to focus on the Pacific, so they were reluctant to start another war in Europe. In those days, Truman was trying to please Stalin, something that put Churchill on nerves.
British analysts finally concluded that if allied finally attacked Russia, they could get quite a success while marching through Germany and Poland. But presumably things would get worser and worser when they went inside Russian territory. The lack of roads and railways in the USSR would make difficult to make a quick advance, and probably, Russian would do the same as in the German attack in 1941: the army and factories would retire to distant places well inside Russia, so British and allies logistic would be strecthed beyond possible. Moreover, British analysts reckoned that the Red Army was much more organised, efficient and motivated than in 1941. Finally, the British public opinion probably won't be prepared to support another war that didn't understand, since the USSR had been an ally against nazis. Churchill was counting with former German forces to support the attack, but this would need a good explanation for the public opinion.
All these consideration made British analysts to reject Churchill's plans.
I'm also conviced that a conventional attack to the USSR would be a complete failure similar to those made by Napoleon or Hitler: distances in the USSR make that they always had a place to retire and reorganize, while attackers are too far away from their supply centers.
Something different is the possibility of a nuclear war in 1946. However, I think it would be very difficult for the British and American public opinion to support a nuclear attack against a non-beligerant country. Some years later, a nuclear war would become a chain of retaliations and winners would be cockroaches.