'Unthinkable' was a British unrealised (and in fact wholly unrealisable) concept for a British and US attack on the USSR (1945/46).

The plan resulted from an instruction of Prime Minister Winston Churchill and was developed by the Joint Planning Staff at the end of World War II in Europe. The undertaking’s primary object was '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.'

The Chiefs-of-Staff Committee was concerned that the great size and strength of the Soviet forces deployed in Europe at the end of the war, and the perception that the Soviet leader Iosif Stalin was unreliable, meant that there existed a real Soviet threat to Western Europe. The USSR had yet to launch its attack on Japan, and so one assumption in the report was that the USSR might instead ally itself with Japan if the Western Allies commenced hostilities. The plan was believed by the Chiefs-of-Staff Committee as militarily unfeasible as a result of the Soviet forces' 3/1 superiority of land forces in Europe and the Middle East, where the conflict was projected to take place.

The main weight of any offensive operation would have been undertaken by US and British forces, as well as Polish forces and up to 100,000 surrendered German soldiers, and rapid success could have resulted only on total surprise. If a quick success could not be obtained before the onset of winter, the assessment was that the Allies would be committed to a total and protracted war. In the report of 22 May 1945, an offensive operation of this type was deemed 'hazardous'. In response to an instruction by Churchill on 10 June 1945, a follow-up report was written about 'what measures would be required to ensure the security of the British Isles in the event of war with Russia in the near future'.

At this time US forces were being relocated to the Pacific theatre in preparation for the planned invasion of Japan, and Churchill was concerned that this 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 USA focused on the Pacific theatre, British odds 'would become fanciful'. The Joint Planning Staff rejected Churchill’s notion of retaining bridgeheads on the continent as having no operational advantage. It was envisaged that the UK would use its air force and navy to resist, although a threat from mass rocket attack was anticipated, with no means of resistance except for strategic bombing.

By 1946 tensions and conflicts were developing between capitalist and communist areas of Europe. These were seen as being potential triggers for a wider conflict. One such area was the Julian March between Italy and Yugoslavia, and on 30 August 1946 informal discussions took place between the British and US chiefs-of-staff about the way in which such a conflict could develop and the best strategy for conducting a European war. Again the issue of retaining a bridgehead on the continent was discussed, with General Dwight D. Eisenhower preferring a withdrawal to the Low Countries, rather than Italy, for their proximity to the UK.