Winston Churchill urged the United States to launch a nuclear attack on the Soviet Union to win the Cold War, a newly released document reveals.

The previously unseen memorandum from the FBI archives details how Britain’s wartime leader made his views known to a visiting American politician in 1947.

Churchill believed a pre-emptive strike on Stalin’s Russia might be the only way to stop Communism conquering the West.

Russian leader Joseph Stalin (left) and Winston Churchill (right) in 1945 - he urged the US to launch a nuclear attack on the Soviet Union to win the Cold War

The note, written by an FBI agent, reports that Churchill urged Right-wing Republican Senator Styles Bridges to persuade President Harry Truman to launch a nuclear attack which would ‘wipe out’ the Kremlin and make the Soviet Union a ‘very easy problem’ to deal with.

The Russians would have been defenceless against a nuclear attack at that time – they did not successfully test their own atomic bomb until 1949.

Britain and the Soviet Union had been allies in the Second World War until 1945, the year Churchill lost office as Prime Minister. But he was one of the first international statesmen to recognise the post-war threat posed by the USSR, and in 1946 made a famous speech in Fulton, Missouri, about an ‘iron curtain’ having descended across Europe as Joseph Stalin consolidated his grip on the eastern half of the continent.

The FBI document shows Churchill’s belligerence towards Britain’s former wartime ally ran so deep that he was prepared to tolerate the deaths of hundreds of thousands of Soviet civilians in a nuclear strike.

Churchill believed a pre-emptive strike on Stalin’s Russia might be the only way to stop Communism conquering the West

The memo claims Churchill ‘stated that the only salvation for the civilisation of the world would be if the President of the United States would declare Russia to be imperilling world peace and attack Russia’.

The note continues: ‘He pointed out that if an atomic bomb could be dropped on the Kremlin, wiping it out, it would be a very easy problem to handle the balance of Russia, which would be without direction.

‘Churchill further stated that if this was not done, Russia will attack the United States in the next two or three years when she gets the atomic bomb and civilisation will be wiped out or set back many years.’

The memo is published for the first time in a book called When Lions Roar: The Churchills And The Kennedys, by award-winning investigative journalist Thomas Maier. It is due to be published in Britain next month. John F. Kennedy regarded Churchill as his hero and made him an honorary American citizen in 1963 – the first person to be given such an accolade.

The two families shared friends, such as Greek shipping magnate Aristotle Onassis, who married Jacqueline Kennedy after her husband’s assassination.

Maier said: ‘Churchill had been a great historian of warfare. He saw the last great cavalry charge during the First World War and championed the development of tanks.

‘I think he saw a nuclear strike as just another progression of conventional warfare, until he realised there was a lot more devastation with nuclear weapons.’