Mr Blair has kept faith with Ms Short
Clare Short's departure from cabinet was an accident always waiting to happen.
Her personal beliefs and her fiery temper often combined to make her unpredictable and explosive.
But, while she once held a particular place in the heart of Labour's left, the way she went has lost her most of that goodwill - and she has left the cabinet with no obvious home to go to.
The agonising over the war - first she was going to resign, then she changed her mind, then she wavered again - dismayed and infuriated her old allies.
And her recent behaviour in missing a cabinet meeting and the crunch vote on foundation hospitals raised fresh questions about the likelihood of her staying.
Ms Short already had a reputation as a serial resigner, but this is far and away her most significant decision.
She has twice before quit the front bench in protest at Labour policies - over prevention of terrorism laws and, notably, the first Gulf war.
She has built her political career on her reputation as a principled straight talker and it has previously both endeared her to the left in the Labour party, and infuriated her leaders.
She is a formidable figure in the Commons and can be famously emotional and occasionally savage.
She certainly has the ability to intimidate those she has taken against and that quite regularly means journalists, who hold a particular place in her heart.
The fact that Tony Blair kept employing her was a constant source of puzzlement in Westminster.
On the wane
Since 1997 she was given a licence to speak and often said things the prime minister wants aired but cannot do for fear of sparking a serious backlash.
A perfect example came at the height of the prime minister's missionary zeal over Africa.
While Mr Blair was talking about the EU working together to aid the continent, Ms Short was blaming the French for helping keep it in poverty.
The MP has resigned before
More recently, however, there have been signs that her popularity with the left has been on the wane.
Leading anti-war campaigner Alice Mahon recently launched a devastating attack on her, suggesting she had long given up expecting Ms Short to put her money where her mouth was.
And there are many in the government and elsewhere who simply think she is too much of a loose cannon to have anywhere near government.
And it is that unpredictability that has probably been her greatest flaw.
The way she issued her threat over Iraq is a classic example.
She had not discussed it with Tony Blair and no one had expected her outburst.
Her passionate beliefs developed early in her life. She was born and bred in Birmingham, the second of seven children of Irish parents.
Her father, a teacher, came from Crossmaglen and she was once strident in her call for British troops to leave Northern Ireland.
With a degree in political science, Clare Short had no notion of entering Parliament until she worked as a private secretary to a Conservative Home Office minister, Mark Carlisle, and found many MPs decidedly "unimpressive" at their jobs.
She thought "I could do that" and in 1983 became the member for Birmingham, Ladywood.
An early target was The Sun's Page Three girl and similar newspaper photographs. The failure of her bill to ban them was inevitable, as was the rowdy reception from some Conservative MPs, which led Ms Short to comment: "If you mention breasts, 50 Tory MPs all giggle and fall over".
Her opinion of another tabloid, The News of the World, plummeted still further five years later, when she complained that it had spent weeks raking through her private life.
"It was unfair, vicious and vile," she said. "They dug out a picture of me at 20 in a nightie. This time I wasn't going to be a man-hating harridan, I was going to be a floozy." Her complaints were upheld.
After calling for the legalisation of cannabis and suggesting that perhaps people like herself could afford to pay more tax, she strained Tony Blair's patience when she refrained from backing his line in a TV interview about tube strikes.
Later she stunned Westminster by introducing her secret son, Toby, to the public, 31 years after she had given him up for adoption.
Pregnant at 18, she had decided with her new husband that they could not look after the baby. Seven years later, the couple divorced, and Clare Short's second husband, the former Home Office minister, Alex Lyon, a victim of Alzheimer's disease, died in 1993.
Recent rebellions include stands over the sale of an expensive military air traffic system to Tanzania and government plans for top-up fees for students.
Ms Short was also the first cabinet minister to condemn Labour's acceptance of a donation from soft porn publisher Richard Desmond.
Where she goes from here is a question that will fascinate Westminster for the foreseeable future.