Operation Unthinkable

The initial primary goal of the operation was declared 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".[2] (The Soviet Union is referred to as "Russia" throughout the document, a metonym that was common in the West throughout the Cold War.)

The chiefs of staff were concerned that given the enormous size of Soviet forces deployed in Europe at the end of the war and the perception that the Soviet leader Joseph Stalin was unreliable, there was a Soviet threat to Western Europe. The Soviet Union had yet to launch its attack on Japanese forces and so one assumption in the report was that the Soviet Union would instead ally with Japan if the Western Allies commenced hostilities.

The hypothetical date for the start of the Allied invasion of Soviet-held Europe was scheduled for 1 July 1945, four days before the UK general election.[7] The plan assumed a surprise attack by up to 47 British and American divisions in the area of Dresden, in the middle of Soviet lines.[7] That represented almost half of the roughly 100 divisions available to the British, American and Canadian headquarters at that time.[5]

The plan was taken by the British Chiefs of Staff Committee as militarily unfeasible due to an anticipated 2.5 to 1 superiority in divisions of Soviet land forces in Europe and the Middle East by 1 July, where the conflict was projected to take place.[8] Most of any offensive operation would have been undertaken by American and British forces, as well as Polish forces and up to 10 divisions of the former German Wehrmacht, remobilised from POW status. Any quick success would be caused by surprise alone. If a quick success could not be obtained before the onset of winter, the assessment was that the Allies would be committed to a protracted total war. In the report of 22 May 1945, an offensive operation was deemed "hazardous".

In response to an instruction by Churchill of 10 June 1945, a follow-up report was written on "what measures would be required to ensure the security of the British Isles in the event of war with Russia in the near future".[10] US forces were relocating to the Pacific for a planned invasion of Japan, and Churchill was concerned that the reduction in supporting forces would leave the Soviets in a strong position to take offensive action in Western Europe. The report concluded that if the United States focused solely on the Pacific Theatre, Britain's odds "would become fanciful".[11]

The Joint Planning Staff rejected Churchill's notion of retaining bridgeheads on the Continent as having no operational advantage. It was envisaged that Britain would use its air force and navy to resist, but a threat from mass rocket attack was anticipated, with no means of resistance except for strategic bombing.