Unthinkable (I am using the plan name here as a by word for any war with the Soviets in immediate postwar Europe, not the actual contingency plan) would have happened only in guise of a major diplomatic crisis with the Soviets involving the plans for postwar Europe. For that to happen, Stalin would need to act very differently then he did IOTL. He'd have to believe that 1) there would be no economic crisis in the West postwar (this is what he assumed would happen), and 2) that any kind of postwar cooperation was impossible. It would involve replacing 1945 Stalin with 1949 Stalin, and therefore deciding that he'd have to use his military advantage at that time if the West did not play ball with him.
I can't conceive of anyway where the Western Allies would attack Stalin unprovoked. The decision was already made by summer 1945 to abandon Poland to its fate. No one was going to start a Third World War over it especially given the relative strength of forces.