It may no longer be unthinkable that 2025 – the 80th anniversary of the end of the Second World War – will be the year when the Special Relationship or even NATO take their last breath. But President Trump would surely have found it harder to break up enduring transatlantic bonds if the last of the generation that fully understood their worth had not left the stage. Without the combined voices of those who fought Hitler’s tyranny, we face an unpredictable future.

Once notable among them was Pamela Harriman, who as a rather gauche teenaged girl making her debut in London society in 1938 was branded a ‘stupid, fat little butterball’ by none other than Kathleen ‘Kick’ Kennedy. Both Kick and her brother Jack – whose father Joe was then serving as US ambassador – were dramatically to change their minds over the course of the war, however. They watched in awe as Pamela transformed herself within a couple of years into a ‘power in the land’.

Kick and the future president both eventually became Pamela’s close friends (and she later went on to support Jack’s presidential campaign). Many others, however, continued to dismiss her as a sex-obsessed gold-digger, someone ‘expert merely in rich men’s ceilings’. They could not know the role of global significance that she was playing behind closed doors.

Only recently, nearly three decades after her death, has it become clear just how Pamela broke free from unpromising beginnings to become one of the 20th-century’s chief unsung power players on both sides of the Atlantic. Her first break came when she became her one-time father-in-law Winston Churchill’s secret weapon in the all-out battle to woo over isolationist American opinion to backing Britain as it fought on largely alone in the Second World War.

Barely into her twenties, Pamela helped to establish deep-rooted ties with the American wartime elite through a campaign of seduction and smarts. While relentlessly pushing Britain’s cause, she gleaned vital intelligence about Washington’s military priorities and political thinking from a roster of strategically selected American lovers, which included generals, key journalists and Washington officials. Churchill was so grateful for this special contribution to strengthening the Anglo-American alliance – what later became known as the Special Relationship – that he paid her a £500-a-year pension for the rest of his life.

Pamela saw these seductions as her patriotic duty. At Churchill’s side in Downing Street as he waged a desperate battle for national survival, she could see that American support was Britain’s only long-term hope of salvation. That experience shaped her thinking for the rest of her life – and even made her yearn to be American. For her (and indeed many of her contemporaries) America held an almost sacred role as global leader – as the Democrat John Kerry later put it, ‘she believes in the larger meaning of our country’. She also knew that America was guided and strengthened by working closely with European allies.

Pamela’s role did not end in 1945, however. It evolved later when as a naturalised American she later helped revive the Democratic Party after its crushing 1980 defeat to Republican Ronald Reagan. Her success in re-energising a deflated party, seeking out winning policies and candidates in winnable seats is currently providing a little comfort to today’s even more demoralised Democrats. What she had learned at Churchill’s side in wartime Downing Street informed some of her thinking on winning popular appeal.

One of the first to champion Bill Clinton as Democratic contender in 1992, she rated not only the small-state governor’s exceptional skills as a communicator, but also valued his interest in the history she had lived. Pamela would leverage her own experience of the Blitz – not least giving birth to her son Winston during a ferocious air raid – to lobby him on the need for American intervention to protect others from aggressors, most notably against the Serbs in Bosnia. She set out to convince Clinton that, to be a respected superpower, the US would have to go beyond its own domestic interests. No-one could forget the relief and hope for a just future when America fully entered the war following the Japanese attack on Pearl Harbor in 1941.

Pamela was born in March 1920, the eldest child of Lord and Lady Digby, but the lottery of being born a girl meant that she would be denied the title and the remaining family estate in Dorset in south-west England in favour of her younger brother. Even a formal education was out of the question. She was raised by governesses at Minterne, a large draughty house in a remote country valley, with the sole expectation of marriage to a fellow but richer aristocrat who might help revive the family’s flagging fortunes.

Pamela yearned for more. A taboo at home on talking politics and especially the prospect of another conflict with Germany, made the subject enthralling to her ever-inquisitive mind. Lord Digby had sustained life-threatening injuries in the Great War and was horrified by Churchill’s bellicose pronouncements on the new threat from fascism – and generally thought the Churchills a bunch of political chancers best avoided.

Equally exhilarating to the young Pamela were the notorious sexual adventures of her 19th-century forebear Jane Digby. Jane’s many conquests had included two European kings, an Albanian brigand and a Syrian sheikh. Pamela’s parents were so concerned by their daughter’s fascination in such scandalous escapades that they burned Jane’s most intimate personal diaries.

Pamela’s interest in politics grew still further when she was sent in 1937 to be ‘finished’ in Munich, where she witnessed with dread SS troops goosestepping in the streets and violent attacks on Jews. She engineered a fleeting encounter with Hitler himself out of curiosity, but came away nonplussed by his implacable hold over the German people.

Soon afterwards, she failed to hook a husband in her first Season, and was the butt of many comments, not just from the Kennedys. She found most of her British contemporaries dangerously complacent and they in turn thought her unladylike and even laughable in her interest in the looming crisis with Germany.

When war broke out, Pamela hastily accepted a proposal from Churchill’s son Randolph on their first date. At that point ignorant of his drunken boorishness and cruel streak, and with few other options, she saw him as an entrée to an exciting world far from the confines of Dorset. Randolph merely wanted a son before he went to war and had already proposed to eight other women, all of whom had wisely turned him down.

Although she was soon pregnant, the marriage was effectively over within weeks, but not before it had vaulted her into Churchill’s closest circle. At once recognising both her quick brain and intense personal allure, he shared with her Britain’s urgent need for American help and quickly came to rely on her for Operation Seduction USA.

Her first charm offensive was deployed on President Roosevelt’s special envoy Harry Hopkins and, as hoped, she rapidly became ‘the apple of his eye’. Harry ‘was a deae’, she recalled ‘but not a pin-up’. Her next target was, however. William Averell Harriman, who was in charge of the Lend-Lease military aid campaign (on which Joe Biden’s aid to Ukraine was loosely based), was tall, dark, very rich and the most sexually attractive man she had ever seen. Randolph was safely overseas and so, in a skin-tight gold lame dress, Pamela choreographed a seduction over dinner at the Dorchester Hotel just as the Luftwaffe began its nightly bombardment. ‘It was a very fortuitous raid,’ she remarked afterwards, while Harriman noted there was ‘nothing like the Blitz to get things going’. By the morning, he was thoroughly recruited to the cause.

Major General Fred Anderson of the US Air Force, the hugely influential CBS correspondent Ed Murrow, his boss Bill Paley, and the American intelligence officer Jock Whitney were among the many others who succumbed over the next three years. Under her spell, all became thoroughly pro-British, and night after night she meticulously extracted useful snippets from them that she directly passed onto Churchill over late night games of bezique. Soon she was also diplomat extraordinaire, using pillow talk to help smooth over Anglo-American spats about the conduct of the war. Even at such a young age, she managed to juggle her various lovers without scandal or embarrassing rows.

She had worked tirelessly for peace but the end of war was challenging. She could not talk about her special operations and nor was she offered anything else now that Churchill was out of power. Divorced, still only 25 with a young son and little money or prospects, Pamela fell back on her sexual powers. She became the mistress of rich men from Gianni Agnelli (heir to Fiat) to the Greek shipping tycoon Stavros Niarchos and Elie de Rothschild. She accumulated frocks, diamonds and a fancy Paris apartment, but what she really wanted was to return to her beloved politics.

Marriage at the age of 40 to the Broadway producer Leland Hayward – responsible for bringing The Sound of Music to a grateful world – brought her to America at last but offered little long-term happiness. With a fading career and failing health, Leland needed a carer more than a wife and soon his – and her – money ran out.

Only on Leland’s death in 1971 was she reunited with her wartime lover Averell Harriman, and they finally married later that year. His status as elder statesman provided another doorway. This time into the heart of the Democratic Party, where she came to outshine her husband in sheer political nous. She transformed his Georgetown mansion into a party hub, gathering rising stars and donors to thrash a way out of the political wilderness. Among her protégés were Joe Biden and Al Gore. Later, even Nelson Mandela would seek her advice before the first multiracial elections in South Africa.

When Clinton finally won the presidency – against the odds – he rewarded Pamela with the American ambassadorship to France, at the age of 73 – her first formal job.

Everything Pamela had learned about handling powerful men now came in useful. She warmed up hitherto frosty Franco-American relations, becoming a trusted personal conduit between Clinton and President Chirac of France and enjoying access that other ambassadors could only dream of.

She deftly smoothed over an embarrassing row over the CIA spying on French officials during trade talks. Her chief contribution, however, was as advocate for intervention against the Serb atrocities in Bosnia. After years of failed peace efforts and the loss of hundreds of thousands of lives, the French hailed her role as ‘invisible but decisive’ in forming the vital western alliance that finally in 1995 bombed the Serbs into submission in the largest NATO military action in history.

Operation Deliberate Force brought together American, French and British forces to spectacular effect, an alliance that Clinton has also acknowledged that she had ‘helped shape’. The ensuing peace accord in Bosnia has lasted until today, however imperfectly. It was a moment – exactly 30 years ago – when NATO and the transatlantic bonds and understandings it is supposed to embody seemed almost unbreakable. It was all part of her oft-stated belief – forged in the horrors of the Second World War and its aftermath – that Washington shared responsibilities with Paris, London and other allies ‘to create a world where peace can rule’ and justice for the vulnerable prevailed.

Yet, despite her successes, the stresses of the job combined with a raft of litigation from her stepchildren – and constant carping from her critics – took their toll. In February 1997, four years after her arrival in France as ambassador, she collapsed while taking her daily swim in the pool at the Ritz Paris. The obituaries were dependably scathing, but those who had witnessed her in action could think of no-one else who had operated so close to the centre of power over 50 years in three countries on two continents.

Chirac gave an emotional tribute in which he hailed her as a ‘peerless diplomat’ who had made the world a safer place. A few days later, a packed funeral at the National Cathedral in Washington heard Clinton give an eight-minute eulogy in which he attributed his rise to the Oval Office ‘in no small measure to her’. Pamela had also been, he declared, ‘an active life force in the greatest continuing alliance for freedom the world has ever known’. The Washington Post was just one voice to question why she was being lauded so lavishly: ‘You would have thought she had done something momentous.’