When Alistair Darling replaced Gordon Brown as Chancellor in 2007, he was confident that his long-standing partnership with the new Prime Minister would stand him in good stead. He knew there would have to be compromises over his Budgets and major decisions with the man who had occupied the post for 10 years; after all, there is always tension between the occupants of 10 and 11 Downing Street. What Mr Darling did not expect was that he would be rubbished in anonymous briefings to the media by people in the Brown inner circle of which he had believed he was a part. The Chancellor is a team player who doesn't have a burning desire to be the captain and doesn't expect team-mates to kick lumps out of each other.
His anger about such briefings has cooled but it is the most plausible explanation for his remarkable admission in a Sky News interview that the "forces of hell" were unleashed against him when he said in August 2008 that Britain faced "arguably the worst" economic crisis for 60 years. He did not intend his Sky interview to make waves and, with hindsight, knows he should not have accepted the premise of Jeff Randall's persistent questioning about alleged briefings against him by the Brown acolytes Damian McBride and Charlie Whelan. If he had only made clear his "forces of hell" were the Tories and the media, Mr Darling would not have fuelled the damaging "Bullygate" affair sparked by claims that Mr Brown bullied Downing Street staff.
The saga had even been running out of steam on Tuesday, raising Labour's prospects of getting back to normal business and focusing on the narrowing Tory opinion poll lead, with several surveys pointing to a hung parliament. Not only did Mr Darling's interview prolong Mr Brown's agony, it also gave more credence to the claims that Mr Brown bullied No 10 staff.
The official version of events is that the Chancellor's explosive remarks were an accident. Another explanation is that they were an accident waiting to happen.
Although the Brown-Darling relationship has not been as rocky as that between Tony Blair and a Chancellor itching to succeed him, it has proved surprisingly bumpy.
It got off to the worst possible start when Mr Brown hijacked Mr Darling's debut, his 2007 pre-Budget report, insisting on a cut in inheritance tax to man-mark a similar Tory pledge because he was toying with an autumn general election. The election was called off and the cut never happened. But the episode made Mr Darling more determined to be his own man, not a grey appendage of Mr Brown. He was egged on by two strong-willed women: his wife, Maggie, a former journalist, and his new special adviser, Catherine MacLeod, a former political editor and close friend of the couple.
That determination led to his candid interview in The Guardian in 2008. Again, Mr Darling didn't intend to rock the boat. His intention was to tell people who he was. His "worst for 60 years" remark was, he says, about the global economy. Again, the timing was awful: it overshadowed an autumn fightback by Mr Brown, who was at a low ebb. The interview included a revealing aside: "There's lots of people who'd like to do my job and, no doubt, actively trying to do it." That was seen as a sideswipe at the Schools Secretary Ed Balls, Mr Brown's closest political ally.
Last spring there was persistent media speculation that Mr Balls would succeed Mr Darling, who was again infuriated by what he saw as an attack from colleagues. But James Purnell quit the Cabinet, leaving Mr Brown too weak to risk another resignation. Mr Darling was offered the Foreign Office, the Home Office and Leader of the Commons but didn't blink, saying it was the Treasury or nothing. Mr Brown backed down. Mr Darling, knowing he was unsackable, grew in confidence.
There was more tension over last year's pre-Budget report. Mr Darling formed an axis with the Business Secretary Lord Mandelson to persuade Mr Brown to acknowledge the need for spending cuts. The Prime Minister agreed the public deficit should be halved in four years but he and Mr Balls emphasised the decision to protect frontline services. Mr Darling and Lord Mandelson used the brief turmoil caused by last month's failed coup against Mr Brown to persuade him to be more upfront about cuts. But a similar battle now looms ahead of next month's Budget.
This week's unintended explosion will reinforce the view among some Brownites that it would have been better to install Mr Balls at the Treasury in 2007. They regard him as a more political animal, more willing to challenge Treasury orthodoxy and thus a better pre-election Chancellor. But the now not-so-grey Mr Darling has shown he will fight to preserve his own reputation and do what is right for the economy, even if that means upsetting the First Lord of the Treasury.