The one Briton without whom VE Day and the freedom of Europe might NEVER have happened

A little after 3pm on May 8, 1945, an open-topped car nosed its way out of Downing Street as an exuberant mass of people surged back and forth along Whitehall. Mounted police tried all but in vain to clear a way for Winston Churchill who was due in the House of Commons to announce formally that the war in Europe was over. Standing up, holding onto the windscreen for support, the Prime Minister held one arm aloft to acknowledge the swelling cheers with his V-for victory salute, a gesture that provoked such a cacophony of delight as almost to drown out the bells of Westminster Abbey that rang out across Parliament Square in celebration.

My father, Richard Dimbleby, already famous as the BBC's first war correspondent, was in Whitehall to describe the scene for a worldwide audience, his voice as elated in peace as it had been intense in war. He understood the feelings of that vast throng, aware that all of them would have powerful memories of the last five years in which their resilience against fear, anguish, sorrow, suffering and loss had been tested to the limit. His own memories were no less powerful.

He had reported from the Maginot Line in the Phoney War, from Egypt as the 'Desert Rats' fought Rommel's Panzers, from an RAF Lancaster at it unleashed its bombs over Berlin, from the skies over Normandy on D-Day, with Montgomery's troops across France, from and a tow-plane over the Rhine as the 6th Airborne Division parachuted into Northern Germany, and few weeks later, from the concentration camp at Belsen-Bergen - a revelatory broadcast that was never to be forgotten. He understood Britain's mood on VE Day.

The crowds in Whitehall had just heard Churchill's prime ministerial broadcast to the nation, his familiar growling cadences booming among them through loudspeakers. Announcing that the German war was over and "the evil-doers... are now prostrate before us", he also reminded them that "our gratitude to our splendid Allies goes forth from all our hearts". And then a cautionary invitation: "We may allow ourselves a brief period of rejoicing but let us not forget for a moment the toil and efforts that lie ahead." His audience barely needed that reminder. They were only too aware that Britain was drained to the point of exhaustion. They knew that much toil lay ahead.

Despite that and although many had family and friends who had not returned and some (still fighting against the Japanese) who never would, they were only too ready to heed Churchill's invitation to rejoice. They sang and they danced in London, and in town centres, and village streets across the nation. Young and old, civilians and soldiers, khaki uniforms and bright frocks, betrothed lovers and total strangers, smiles and laughter, hugs and kisses mingling with tears of relief at many thousands of impromptu parties that lasted through the night.

For the prime minister, though, it was a bittersweet moment. He was already brooding on a darkening future. So much so that, within days of those celebrations - astonishing as it may now seem - he instructed the Chiefs of Staff to draw up a plan for a pre-emptive military strike against one of his "splendid Allies" - the Soviet Union - which, with a death toll of some 27 million people, had suffered far more than any other to destroy the Nazi menace. The Chiefs did as they were told.

A fortnight later they gave their verdict, reporting in effect that "Operation Unthinkable"(as it was codenamed), would be a Mission Impossible. It was duly kicked into the ashcan of history. Yet the foreboding which had animated Churchill's bizarre instruction, was deepening.

He originally hoped that the buffer states around the Soviet Union, which he and Roosevelt had approved, would constrain Stalin's westward march and allow the re-establishment of independent states in the rest of Eastern and Central Europe. But, by the summer of 1944, as the Red Army advanced deeper into Poland, those hopes were shattered.

While Roosevelt still believed that his rapport with 'Uncle Joe' would form the basis for a post-war partnership through which (together with Britain and China) they would together preside genially over a world of nation states in peace, security and freedom, Churchill took a diametrically different view. In that summer, in one of several similar cris de coeur, he wrote in anguish to his foreign secretary, Anthony Eden: "I fear that a very great evil may come upon the world... the Russians are drunk on victory."

Though he contributed effusively to the deluge of warm words which spewed out of the Yalta Conference in February 1945, the formal agreement between the Big Three (which did little more than codify the secret understandings reached at their Tehran Summit in late 1943), confirmed the truth of the adage that "to the victor goes the spoils". Churchill knew that and he abhorred the prospect. But there was now little he could do.

He had long realised, as he put it, that, as British prime minister, he was "the poor little English donkey" squeezed between "the Great Russian bear" and "the Great American buffalo". By VE Day, Britain was weaker than ever. Moreover, within two months, Churchill was to pay the democratic price of wartime victory, ousted from power in a landslide vote for the Labour Party which the voters judged to be more likely to meet their peacetime aspirations.

But that was for the future. On that evening 80 years ago, my father was in the Mall, with the ever-growing, ever more exuberant crowds, when, to everyone's surprise, Churchill, in his boilersuit, walked out onto the floodlit balcony of the Ministry of Health. Taken aback, the broadcaster was almost lost for words in his excitement.

"Oh what wonderful luck! How wonderful! How wonderful," he exclaimed, stopping just in time as Churchill began to speak. "My dear friends, this is your hour. This is not the victory of a party or of any class. It's a victory of the great British nation... We were alone for a whole year... Did anyone want to give in?" "No!" yelled they yelled back.

"Were we downhearted?" "No!" bellowed the crowd. And then, as he said "God bless you all", the band started to play Land of Hope and Glory. The crowd started to sing, the sound swelling, Churchill began to conduct, as Dimbleby, reflecting the mood, told his listeners: "This suddenly has become a very moving moment." As Elgar's hymn to patriotism ended, the mood changed again as the crowd followed up with a raucous rendition of: "For he's a jolly good fellow."

At that moment a visibly moved Churchill must have known that the nation's gratitude to him for steering Britain through those dark years was as profound as it was to prove to be lasting.

• Jonathan Dimbleby is the author of Barbarossa: How Hitler Lost The War; and Endgame 1944: How Stalin Won The War, both published in paperback by Penguin, priced £12.99 and £10.99 respectively