Jackie Kennedy captivated the nation as America’s very own princess, embodying grace and sophistication during her tenure as First Lady. Her poise enchanted everyone she encountered.

While Jackie maintained a dignified distance from controversy, her charismatic husband, President John F. Kennedy, was known for his numerous affairs, which included liaisons with friends’ wives and Hollywood icons like Marilyn Monroe and Anita Ekberg.

However, recently unveiled letters have disclosed that Jackie found both revenge and solace in a profound and personal relationship with Robert McNamara, one of her husband’s trusted advisers and a pivotal figure behind the Vietnam War.

These letters, kept under wraps until now, offer new insights into not only the Kennedy administration but also the dynamics of the Kennedy marriage.

Publicly, McNamara, the Secretary of Defense, was perceived as a detached and calculating figure, heavily focused on military statistics, and regarded as someone who viewed American troops as mere cogs in the war machine.

His Washington nickname was ‘the Brain’.

But Jackie saw a very different side to him. Authors Philip and William Taubman reveal, in their book McNamara At War: A New History, that this unlikely romantic liaison began in 1962 with a dance fad that was sweeping the country – the Twist.

At a White House party on February 9, the President provoked gossip by slipping out of the room for an hour or so.

Jackie Kennedy – America’s princess – bewitched all who met her

JFK was renowned for his womanizing, but it now emerges Jackie had a long, intimate relationship of her own outside the marriage

Among the President’s rumored affairs was a well-publicized one with Marilyn Monroe

Mary Meyer, the sister-in-law of a prominent political journalist, also vanished. The truth was obvious: they were having sex in the family rooms upstairs.

JFK’s sexual appetites were an open secret: ‘If I don’t have a lay for three days,’ he said, ‘I get a headache.’

That night, however, Jackie rose above the rumors by dancing the Twist – considered shockingly provocative at the time – with McNamara, partying until 3am.

Society columnist Betty Beale reported breathlessly: ‘Anyone who still had any misgivings about the current dance craze simply hasn’t seen it done the way Mrs Kennedy, who looked lovely in a long white satin sheath, and Secretary McNamara performed it.

‘It was rhythmic, fun and peppy.’

The coverage delighted Jackie, who created a cheeky Valentine’s Day collage for her dance partner the following week.

Snipping out a magazine picture of a couple doing the Twist, she pasted photos of her face and McNamara’s onto the bodies and added four pages of newspaper cuttings about their antics at the party.

‘Bob McNamara proved to be the most agile of the Cabinet twisters,’ reported the Washington Post.

The President couldn’t work out why any woman would see sex appeal in McNamara

McNamara was regarded by the public as a cold fish, but Jackie saw something else in him (pictured: the defense secretary with his wife, Margaret)

The Twist was considered shockingly provocative at the time

She arranged for the collage to be delivered by hand, in an envelope with the White House crest.

A few days later, she passed on some snapshots of them, taken at the party, with a note joking that she would have to ‘invoke executive privilege’ – that is, to insist that the First Lady was above the law – if hardline Republicans ever saw the photos.

The flirtation quickly became more serious. Jackie, frustrated by the need to remain aloof from her husband’s political battles, took a keen interest in McNamara’s handling of the Cuban Missile Crisis in October 1962, when America stood on the brink of all-out war with the Soviet Union.

On October 25, as the crisis reached its height, she sent him a handwritten note of sympathy, telling him she knew how tired he must be. It was difficult to find the right words, she said, to express how grateful she was that he was involved in searching for a solution.

‘Please know that this is not said as Jack’s wife – but just as a plain person.’

However, she added some gossip that could only have reached her as First Lady. No ‘plain person’ could have heard the story that Secretary of State Dean Rusk had been discovered one morning asleep beside the desk of a presidential aide, so comatose that one hand was lodged in a potted cactus plant.

JFK was aware of his wife’s admiration for his Secretary of Defense, if somewhat nonplussed by her choice of confidante.

He told political correspondent Ben Bradlee that, as a man, he couldn’t work out why any woman would see sex appeal in McNamara.

The authors speculate that Jackie was drawn to McNamara’s strong spiritual and romantic streak 

Snipping out a magazine picture of a couple doing the Twist, Jackie pasted photos of her face and McNamara’s onto the bodies

She added four pages of newspaper cuttings about their antics at the party

The collage was then hand-delivered to McNamara, in an envelope with the White House crest

It certainly wasn’t his prosaic past. McNamara was a former US Army Air Force captain who spent most of World War II in the Office of Statistical Control, before becoming a Ford Motor Company executive.

By the time he was headhunted for the Kennedy administration, he was Ford’s president. Colleagues found him difficult to know, cold and unemotional. ‘You could never really get to know Bob, or put your feet up with him,’ said one.

But he had a romantic streak that very few saw. Remembering the Valentine’s collage Jackie had sent in 1962, the following year he sent her a handmade card on February 14.

She seemed genuinely thrilled, responding with a card on White House paper, on which she drew a heart with an arrow through it.

His ‘jolly’ billet-doux was ‘quite fantastic’, she told him, and deserved to be an exhibit in some future Kennedy museum.

Then, teasing him, she said her husband’s military aide Godfrey McHugh had also received an unsigned Valentine’s card. Had McNamara sent that too?

When Kennedy flew to Berlin that spring and gave his celebrated ‘Ich bin ein Berliner’ speech, Jackie and McNamara watched it together in the family quarters at the White House.

Jackie, who was heavily pregnant at the time of the speech, enthused about her husband’s oratory which, she said, was much improved.

The next evening, she and McNamara went to dinner together at Salle du Bois, a discreet restaurant in Georgetown, Washington DC.

Their conversation must have been esoteric, because before returning to the White House Jackie sent him a note to thank him for the gift of a book about mysticism, Arthur Koestler’s The Lotus And The Robot.

The book, battered and well-thumbed, appears to have been McNamara’s own treasured copy.

Jackie told him she admired his ‘humility’ and looked forward to a long exchange of ideas on religious topics. It was this spiritual side that appealed to her most in their friendship.

McNamara’s fondness for Jackie was earthier. One acid-tongued acquaintance noted that the Secretary of Defense became ‘incandescent’ whenever he danced with the First Lady.

Since 1940, McNamara had been married to Margaret, or Margy, whom he’d courted in his teens, but they had drifted apart and she frequently visited friends alone.

Margy also didn’t believe that her husband’s friendship with the First Lady was entirely elevated and platonic. While he was dining at Salle du Bois, she tried repeatedly to phone him.

Friends said she became ‘visibly bothered’, until finally she tracked him down.

‘Well, I reached him!’ she announced bitterly.

JFK was aware of his wife’s admiration for his Secretary of Defense, if somewhat nonplussed by her choice of confidante

McNamara helps Jackie out of her car in May, 1967, more than three years after JFK’s assassination

McNamara’s wife Margy (photographed together in 1974) wasn’t convinced her husband’s friendship with the First Lady was entirely platonic

A few weeks after that dinner, Jackie gave birth to a baby boy, Eric. He died a day later.

Philip and William Taubman speculate on what might have attracted Jackie to her unlikely suitor.

‘Both McNamara and Jackie seemed to yearn for companionship outside their marriages, even intimacy, if more of the soul than the body,’ they write.

‘Unlike many of the men in the Kennedy administration and family, including the President, McNamara radiated respect for women and an interest in their lives and intellectual pursuits.

‘He could be a sensitive companion with them, a sympathetic listener.’

They add: ‘For Jackie, an offstage friendship with McNamara may have offered an escape from the glare of the White House and the intensity of the Kennedy clan and its preoccupation with politics and competitive sports.

‘Here was a confident, powerful, and reliable man, yet in her company a surprisingly vulnerable and empathetic figure who could be trusted to keep secrets and favored some of the same books and music that she did.

‘He seemed to her to be compassionate about other people’s suffering; strong and decisive, brilliant but not a show-off, capable of sweetness and sorrow.’

However, their friendship reached a new level of intimacy on November 22, 1963, when John F Kennedy was assassinated by sniper fire as he drove with Jackie through Dallas.

Even before the autopsy was carried out, Jackie begged her brother-in-law Robert to send for McNamara. He flew out from Washington DC and was at her side throughout the examination of the body, before returning to the capital with the newly widowed First Lady.

‘I got to the hospital at 7.45pm and she talked for the entire time until we left for the White House, arriving there at 4.25am,’ he recalled.

‘She was in that suit with the bloody skirt and blood all over her stockings.

‘I felt I had to be calm for her and listen to her. I was concentrating entirely upon her, because she needed me and I felt, the hell with the others, let them take care of themselves.’

Deeply moved by her grief, and wishing to do something to console her, he offered to buy the Georgetown house where she and Kennedy had lived after their marriage, as a present.

He continued to give all the emotional support he could, earning the thanks of the wider Kennedy clan. Months later, Bobby Kennedy said to McNamara in a handwritten note: ‘You have carried the burden so well for so long. And for this we are all grateful.’

For the next few years, their friendship settled into a pattern of regular, discreet assignations. They would meet in New York, dining at La Caravelle, while Margy was out of town.

During a visit to Vietnam, McNamara was presented with a stuffed tiger as a gift by Premier Nguygn Cao Kh. He gave it to Jackie’s children, Caroline and John Jr – mindful that she had once suggested he could be a role model to her son.

By now, America was deeply embroiled in the Vietnam war. Many saw McNamara as one of the hawks who wanted to intensify the campaign against the Communist Viet Cong.

But Jackie believed he was a peacemaker at heart, though she sometimes accused him of doing too little.

One evening at her Manhattan apartment overlooking Central Park, after dinner, when they were discussing the Chilean poet and Nobel prize winner Gabriela Mistral, she turned on him.

‘Whether her emotions were triggered by the poem or by something I said,’ he told a friend, ‘I do not know. She had grown very depressed by, and very critical of, the war.

Robert McNamara was at Jackie’s side throughout the autopsy of the assassinated President, before returning to the capital with the newly widowed First Lady (McNamara is behind the white fence at the graveside)

After Jackie married Aristotle Onassis, McNamara continued to write to her, but her replies became brief and formal

‘In any event, she became so tense that she could hardly speak.

‘She suddenly exploded. She turned and began literally to beat on my chest, demanding that I “do something to stop the slaughter!”‘

Philip and William Taubman never state outright that Jackie’s relationship with her husband’s trusted adviser was a sexual one.

And neither Jackie nor McNamara ever admitted to a physical affair with each other.

But it is very difficult to place any different interpretation on the facts. Though they clearly enjoyed an intellectual kinship, and shared a deep interest in spiritual questions, it seems impossible that, as a widow, Jackie would have invited this married man back to her apartment simply to continue their dinner conversation – not just once but many times.

Indeed, after she married Aristotle Onassis in 1968, their relationship waned. McNamara continued to write to her, but her replies became brief and formal.

Margy died in 1981 and McNamara eventually remarried, a few years before his death in 2009.

Jackie died in 1994. A year earlier, in a flash of the old passion she once felt for McNamara, she sent him a brief note.

It read: ‘You will always be my shining knight.’

McNamara at War: A New History by Philip Taubman and William Taubman is published by AA Norton and Co