On May 22, 1945 -- twelve days after VE Day -- British Prime Minister Winston Churchill directed the British Chiefs of Staff to develop a contingency plan for a war against the Soviet Union. The plan, which Churchill personally codenamed Operation Unthinkable, was to be based on the assumption that the alliance between the Western Allies and the Soviet Union had broken down and that conflict was imminent. It was, by any measure, one of the most extraordinary documents ever produced by the British military establishment.
The plan envisioned a coalition force of British, American, and Polish troops, reinforced by up to 100,000 recently surrendered German soldiers (the plan acknowledged that these troops would be fighting against their former allies and would need careful handling), launching an offensive from Western Germany toward Berlin and Warsaw. The objective was to push the Red Army back to the pre-war Polish border and restore independent governments in Poland, Czechoslovakia, and Hungary.
The planners, who produced their report on June 8, 1945, were brutal in their assessment. The first line of the report read: "The possibility of a war with Russia is one which must be faced by the British Empire and the United States of America." The report went on to estimate that the Western Allies would need to deploy approximately 47 divisions to achieve their objectives -- a force that did not exist and could not be assembled without calling up additional conscripts and stripping garrison forces from across the empire. The report noted that the atomic bomb, which the US had used against Japan the previous August, could be used against Soviet forces, but that the current American stockpile was small and that the political consequences of using nuclear weapons against Russia would be catastrophic.
The report's conclusion was unambiguous: the plan was, as its name suggested, unthinkable. The Soviet Union had a massive advantage in conventional forces in Europe, and any attempt to push them back would require a degree of military commitment that Britain, still exhausted from six years of war, could not sustain. Operation Unthinkable was quietly shelved. It was never presented to political leaders as a serious option.
What Operation Unthinkable reveals is the depth of the distrust that already existed between the Western Allies and the Soviet Union within weeks of their shared victory over Nazi Germany. Churchill, who had spent the war building an alliance with Stalin out of strategic necessity, was already thinking about what came next. The Cold War, which would define the next forty-five years of world history, had already begun in the mind of the British Prime Minister before the war in Europe was a fortnight old.
The full text of Operation Unthinkable remained classified until 1998, when it was released by the British Public Record Office. It can now be read in full at the National Archives in Kew, London.