When the end came, it was vicious and vituperative. Six days before the hotly disputed events aboard Brad Pitt and Angelina Jolie’s private plane – when the allegedly inebriated actor is said to have verbally abused his 15-year-old son Maddox – Angelina said she wanted to talk in the airy living room of their sprawling mansion in the Los Feliz hills, with its panoramic views of Hollywood far below.

With their marriage in turmoil, the 41-year-old actress had, she said, worked out a plan. She would take their six children to live in Britain – increasingly the base for her charitable and political work as a United Nations Special Envoy for Refugees – while Brad would remain in Los Angeles to work on his film career.

To the outside world they would remain a happily married couple, she added. But each would be free to pursue whatever liaisons took their fancy. Furthermore, Brad would be able to see the children whenever he wanted to.

The couple are said to have rowed over Angelina’s suggestion that they have an open marraige

When Brad said his wife’s proposal was absolutely unacceptable, the famously volatile Jolie flew into a terrifying rage. In an instant, all the many fault lines in their tempestuous relationship were laid bare.

There were, friends say, allegations of infidelity on both sides, poisonous barbs from each of them about the other’s obsession with work. Most furious of all was the screaming row about the fate of their children, to whom Pitt is said to be devoted.

As has happened many times before, Brad is said to have struggled to cope with the sheer, frightening force of Angelina’s volcanic temper. He retreated to his room and locked himself in, leaving her beating the door with her fists and shouting for him to come out.

At the heart of the extraordinary confrontation was Angelina’s burning desire to move to Britain and devote herself to her wholly laudable political career, which has seen her campaign against violence against women in war zones, for the education of women in the developing world, and to ease the plight of the world’s millions of refugees.

She wanted to set up a permanent base at the couple’s rented £15,000-a-month home in Surrey, send the children to school in Britain, and pursue her ambition to continue the work of the late Princess Diana, whose life and role of champion of the landmine victims and AIDS sufferers she has studied in minute detail.

Furthermore, she has become obsessed with emulating her friend and mentor, Baroness Arminka Helic, by gaining a seat in the House of Lords as a platform from which to widen her campaigning work, and is prepared to give up her American citizenship to do so.

Angelina is said to want to be the new Princess Diana, having studied her work in great detail

One friend even suggested that Angelina harboured a secret ambition to marry into the Royal Family, saying: ‘Prince Andrew is on her radar.’

When the family travelled to France last week, Brad believed the storm had passed. But Angelina was merely biding her time.

She had secretly consulted her lawyers, who told her there was only one way she could ever execute her plan: she needed to make sure by any means possible that she gained full legal custody of the children.

The best way to do that, Angelina decided, was to drag her husband’s name through the mud in the most damaging way possible by alleging he had harmed the children. Hence the headlines that Brad had physically abused Maddox on the return flight after he ‘got drunk and went wild’.

Brad’s friends say the actor was ‘crushed and devastated’ when he was hit by the double whammy of being served with divorce papers and simultaneously by allegations of drunkenness, violence to his son and an affair with French actress Marion Cotillard – rumours she has denied.

American newspapers have also reported that Angelina was fed up with her husband’s marijuana use. Friends say Brad may indeed have been drunk on the flight (and the FBI, which has jurisdiction over offences committed in mid-air, is making inquiries into the episode to see whether there is a case to answer), but would never have hit any child, let alone his son.

‘Brad is a pacifist,’ says one of his oldest friends. ‘He always shied away from confrontations with Angelina and is a devoted father. He is the one who plays with the children, not her.’

The friend adds that Brad was determined not to follow Angelina into the mud-slinging gutter.

‘He has always said he wants to take the moral high ground. ‘Whenever they have had “sell the house” arguments before, he has always pressed for them to go into mediation, divide their assets amicably, and agree to share the custody of the children before announcing to the world that they had gone their separate ways.

‘That’s why Brad feels so bereft and betrayed now. He can’t believe she is trying to trash his name and reputation because he has always said, despite the advice of his lawyer, that he would never bad-mouth the mother of his children.’

Despite the seismic shock among their Hollywood friends at the sudden and bitter nature of their split, few who – like me – have studied the evolution of the Brangelina phenomenon for more than a decade will be surprised that they are going their separate ways.

One friend suggested that Angelina harbours a secret ambition to marry into the Royal Family,

The relationship has been dead in the water for the best part of a year. Close friends of the couple to whom I have spoken in recent days told me they frequently slept in separate wings of their huge home.

And it was recently revealed that Angelina had rented a £50,000-a-month five-bedroom home in Malibu in readiness for her new Brad-free life.

Angelina’s tantrums have been an ever-present leitmotif of their relationship, with rows typically ending with Brad locking himself in his room, unable to cope with her. I said in 2009 that I would be surprised if Brad and Angelina were still together in seven years’ time.

It was a prediction based on their friends telling me during research for my book on the couple that they had decided the Brangelina brand was useful and profitable for both of them.

The friends told me that they had agreed then, at a time when they were seldom off magazine covers, that most showbusiness brands had a seven-year shelf life before the public became bored and moved on. This understanding and an intense physical connection helped paper over the cracks in their relationship, friends say.

But behind this glamorous facade, trouble was never far away.

The major fault lines included money. Although they are together worth hundreds of millions of dollars, much of their wealth is tied up in property and in Brad’s film projects, with the result that they often spent way beyond their means.

When they went out, for example, they would typically hire an entire restaurant for their family and entourage, paying up to three times the venue’s typical nightly income for the privilege.

This, along with a fleet of limousines and comprehensive security arrangements, could cost as much as £65,000 for a single meal.

Angelina has been pictured kissing her brother on the red carpet at ceremonies

Another point of conflict was the role played by Angelina’s brother James Haven, whom she was memorably pictured kissing on the lips on the red carpet at the Oscars, prompting her to deny their relationship was incestuous.

Haven is said to be paid ten per cent of her earnings to act as the family’s full-time nanny – a role Brad is said to resent.

They rowed, too, about the way that Angelina liked to drag the children around the world. Brad, and his worried parents, believed the children should be given a more settled upbringing with regular schools and the friendships that brings.

Such was the stress of the relationship that Brad took to drinking, friends say, and Angelina struggled with an eating disorder that saw her become dangerously underweight two years ago.

Nevertheless, the marriage endured. Indeed, contrary to recent speculation, Brad’s friends insist that he has never regretted leaving his former love, actress Jennifer Aniston, for Angelina, if only because he adores being a family man with six children. His only regret, they say, was that he didn’t do more ‘due diligence’ on Angelina as he had no idea at the beginning of their relationship that she was prone to violent mood swings.

But now, their friends maintain, Jolie has made the momentous decision that her life should take a radically different path – one that leaves the film industry behind. Angelina, sources add, has long felt a deep sense of inferiority when she compared her Hollywood career – which she feels stalled after she won a Best Supporting Actress Oscar for Girl, Interrupted in 1999 – to her husband’s much-garlanded and hugely successful body of work and reputation as one of the most acclaimed screen actors of his generation.

Some say Brad believed she was being paranoid when she complained she had effectively been blacklisted by the Hollywood studios, and point out that he has always been devoted to his craft.

He continues to take acting lessons to this day and seeks to improve his performance with every film appearance.

Jolie, however, began to imagine a different life as a politician.

She first tried to persuade Brad to emulate Ronald Reagan by leaving his acting career behind to launch himself on the road to the White House, with her as the First Lady.

Brad’s friends insist that he never regretted leaving his former love, Jennifer Aniston, for Angelina, if only because he adores being a family man with six children

But while Brad has frequently supported liberal and Democratic causes, it is said he has no interest in a political career.

His reluctance to enter politics is said to have infuriated Jolie, who set out to create her own power base by campaigning on humanitarian issues.

Even those friends who are wary of her volcanic temper are convinced by her sincerity and sheer hard work in her campaigning. But it is clear that her ideas have gained more traction in Britain – where she was made an honorary dame in January 2014 – rather than in her native America.

According to my sources, Jolie is as charming and hardworking as she is ambitious.

She is also an inveterate networker who became close to David Cameron and former Foreign Secretary William Hague.

However, her closest pals and mentors in Britain are Baroness Helic, who fled the war in her native Bosnia in the 1990s and became an adviser to William Hague, and Chloe Dalton, another former Hague aide. Baroness Helic runs Jolie’s charity, and she and Dalton are said to advise her on everything from foreign travel to childcare.

It was Helic who first made Hague watch Jolie’s harrowing 2011 film In The Land Of Blood And Honey, a love story set against the backdrop of the conflict.

‘Angelina has become convinced that she can become something like a cross between Princess Diana and Mother Teresa,’ says one long-term confidant.

‘She has studied Diana’s life in minute detail and thinks she can carry on her work.

‘She is also prepared to give up her American citizenship to become a member of the House of Lords, which she thinks she can use as a platform to push her project forward. She has talked about it for years.

‘Don’t underestimate her – she is utterly charming and absolutely persuasive with people who are in positions of power.

Networker Jolie is close to William Hague – she’s pictured here in 2012 with the then Foreign Secretary ahead of a screening of her film ‘In the Land of Blood and Honey’ about Bosnia

‘She has always got what she wants and always moves onwards and upwards, just as she did from [former husbands] Johnny Lee Miller to Billy Bob Thornton to Brad Pitt.

‘Now she thinks she’s outgrown even him. I think she wants to emulate Grace Kelly by moving seamlessly from Hollywood to European royalty.

‘Her goal now is definitely to hook up with a high-profile politician or a Royal. She thinks she fits Prince Andrew’s profile and I believe he is on her radar.’

It all leaves heart-throb Brad having to cope with the unfamiliar humiliation of suddenly being surplus to Angelina’s requirements.

He also faces an agonising decision: does he defy the advice of his lawyers and insist for the good of his children that he will continue to inhabit the moral high ground by staying silent about Angelina’s split personality.

Or will he swallow his principles and fight fire with fire?