Operation Unthinkable: Churchill’s Postwar Contingency Plan | History Hit

Operation Unthinkable: Churchill’s Postwar Contingency Plan

Amy Irvine

21 Apr 2023
Churchill, Roosevelt, Stalin at the Yalta Conference, February 1945

‘Operation Unthinkable’ was a secret military plan proposed by Winston Churchill and developed by the British Armed Forces and the USA in May 1945. It aimed to launch a surprise attack on the Soviet Union and force Stalin’s communist government to retreat from Eastern Europe, stopping the spread of communism.

The plan, however, was ultimately deemed unfeasible and did not go ahead.

Why was such an audacious plan even considered so close to the end of such a mighty conflict as World War Two, especially against a country that had been a significant and valuable ally?

Growing concerns

Operation Unthinkable was born from Churchill’s growing concerns over the Soviet Union’s intentions in Europe.

Recognising that they were on their way to defeating the Axis powers, the Allied leaders met at the Yalta Conference in February 1945 to discuss how to re-divide and distribute Europe after the war.

Stalin was deeply suspicious of the other powers, believing they had delayed the Allied invasion of Italy and the invasion of Normandy to cause the Soviet Army to struggle alone against Nazi Germany, to wear each other down.

Russia had lost over 25 million citizens and a third of its national wealth during World War Two, prompting Stalin to believe he had every right to take as much European territory as a prize as he could get away with. Stalin had virtual control of Poland, and was setting up a provisional, communist, pro-Soviet government. (He also had 2.5 million Soviet soldiers around Berlin and Eastern Germany.)

Despite his Yalta Conference pledges of free and fair elections in Poland, it became clear Stalin later reneged on these.

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Churchill was determined fair elections were held, viewing the spread of communism as a threat to British interests in Europe and believing a military confrontation with the Soviet Union increasingly inevitable. Churchill had also been alarmed by reports of systematic rape and destruction by the Soviets in Berlin and occupied territories, with infrastructure stripped and sent back to Russia.

However, Roosevelt, being anti-imperial, suspected this was part of Churchill’s ambition to maintain the British Empire’s influence; America wanted to maintain Soviet cooperation to help defeat Japan. Roosevelt pointed out that whilst allies, the US were not there to help Churchill hang on to his “archaic, medieval empire”.

By 8 May, VE Day, Western forces had pushed around 150 miles beyond the Yalta agreed boundaries to a line of contact with the Soviets. Whilst the US were keen to relinquish this territory, Churchill wanted to use it as a bargaining chip, warning against retreating until the Allies were satisfied about Poland and the Russian occupation of Germany.

Thus in early May 1945, Churchill commissioned a contingency military enforcement operation plan for war on the Soviet Union to obtain a “square deal for Poland”.

Allied army positions on 10 May 1945

Image Credit: Wikimedia Commons / W. B. Wilson / CC BY-SA 3.0

Operation Unthinkable

Codenamed Unthinkable, the plan called for a massive surprise assault on Soviet positions in Europe by British and American troops, with support from German and Japanese forces (rearming between 100,000-700,000 former Wehrmacht soldiers), and to ‘impose upon Russia the will of the United States and the British Empire’.

Suspicious of Stalin and to pre-empt any potential future Soviet attack, the aim was to push Soviet forces back beyond the Oder River and establish a new pro-Western government in Poland, hoping this would force Stalin to reconsider his domination of Eastern Europe. The plan would need to be executed quickly while Britain still had its military force mobilised.

The date was provisionally set for 1 July 1945, but while British planners anticipated America would have to be involved, at this stage they were not consulted. Since Roosevelt’s death on 12 April, President Truman and the State Department had continued to see Churchill as an irritation in their dealings with Stalin.

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Scepticism and opposition

Although only known to Churchill, his three Chiefs of Staff and the immediate planning team, the plan was met with a great deal of scepticism and opposition. They saw it as unrealistic, and worried it would lead to a prolonged and costly conflict with the Soviet Union.

Once informed of the plan, the USA also opposed it. America were focused on rebuilding Europe and promoting democracy. They did not see the Soviet Union as a significant threat, and worried a military conflict with the Soviet Union may well prompt the Soviets to then ally themselves with Japan.

Churchill continued to push for Operation Unthinkable’s implementation, yet it increasingly became clear the plan was unfeasible and would not achieve its intended goals.

Without America’s support, it was difficult to launch a full-scale military operation. The plan also relied on the support of former German soldiers, and the British military believed the British, US and Polish forces may not have been willing to operate alongside their recent enemy.

The Soviet Union’s scale meant it would have been difficult to deploy Allied forces in a way that would catch them off guard. The Soviet Union also had a powerful military (on 1 July 1945, the Red Army numbered around 7 million men, of whom 6 million were in the western theatre) and would have been able to put up a strong defence. Planners calculated that Soviet infantry divisions exceeded the UK’s by 4:1, and Allied air power had greatly diminished.


While the plan was initially approved by Churchill and scheduled for 1 July 1945, on 8 June 1945 the Chiefs of Staff submitted their final conclusions. Due to the huge numerical superiority of the Red Army, it was thought a short, sharp Allied attack into Poland would fail, leaving total war the only outcome.

Concerns that the Soviet Union could respond with a nuclear attack deemed Operation Unthinkable too risky, and it did not go ahead. 

At the Potsdam Conference, news of the successful test of America’s atomic bomb reignited Churchill’s hopes of Operation Unthinkable, but it was America’s bomb. Clement Attlee became Prime Minister in July 1945, and with a new government, Operation Unthinkable was abandoned.

It’s unknown if Stalin knew anything about Operation Unthinkable, but he did know the British were stockpiling German weapons and supplies, and may well have received secret British documents relating to Unthinkable, probably via Donald Maclean, later exposed as one of the Cambridge spies.

Sir Winston Churchill (right); Clement Attlee (left)

Image Credit: Public Domain, via Wikimedia Commons

8 months on from his 1945 general election defeat, on 5 March 1946 Winston Churchill delivered a speech in Fulton, Missouri in President Truman’s presence, including the phrase ‘iron curtain’, which would come to describe countries living within the Soviet sphere of influence. Churchill, an unashamed imperialist, helped paint Russia as a dangerous expansionist power, which did not respect ‘military weakness’ and needed to be strongly dealt with. His words set a major precedent in post-war relations between the Western powers and the Soviet Union.

Within months, US Chiefs of Staff, alarmed at the extent of Soviet expansion, began work on their own plans for a war with the Soviet Union. Liaising with their British counterparts to discuss how best to meet the challenge, they devised ‘Operation Pincher’. Thus by 1946, Operation Unthinkable was not quite the outlandish plan it seemed one year prior. The Cold War had begun.


Despite never being implemented, Operation Unthinkable is historically significant. It was the first Cold War-era contingency plan for war against the USSR, and reflects the tensions and mistrust that existed between the Western Allies and the Soviet Union in World War Two’s aftermath. It also provides insight into the complex geopolitical dynamics of the time and the challenges of developing effective strategies to counter the spread of communism.

Operation Unthinkable was not made public by the Cabinet Office until 1998, 7 years after the Cold War ended. Despite the document (reference CAB 120/691) being released, maps and plans were never brought into the public domain – either destroyed or still withheld.

Amy Irvine