In May 1945, on the order of Prime Minister Churchill, the British Military created a war plan called Operation Unthinkable which was based on the hypothesis that Hitler’s defeat marked a hiatus in the Second World War and not the end, and that there could soon be a war with the Soviet Union. The operation’s primary goal was stated as "to impose upon Russia the will of the United States and the British Empire. Even though 'the will' of these two countries may be defined as no more than a square deal for Poland, that does not necessarily limit the military commitment”
A war with the Soviets was clearly thinkable to Churchill and his government. The British planners were concerned about the massive deployment of Soviet troops in Eastern Europe—the Red Army outnumbered the Allied Force four to one, and they had better tank deployments. The idea was to have a strategy in place to defend the British Isles in the event of a war. In the initial versions of the operation, the conclusion was reached that without massive American support, it would be hard for Britain to defend itself against a full-fledged Soviet attack. In the later versions of the operation, a strategy was developed to defend the British Isles against Soviet drive towards the North Sea and the Atlantic after the American forces were forced by the Red Army to withdraw from the continent.
Operation Unthinkable was a top-secret during the Cold War. Even the USA was not informed of its existence. The operation was made public in 1998, seven years after the Soviet Union fell. But the British spy Guy Burgess had leaked to the Soviets some initial details regarding British contingency war plans when he defected to the Soviet Union in 1951.