David Miliband was the golden boy of New Labour for so long that it began to seem inevitable he would one day lead the party.
His announcement today that he is stepping back from frontline politics robs Labour of one of its biggest talents and weakens the Blairite presence in a party which has already made a shift to the left with his brother Ed's election as leader.
With his intensely political background as the son of Marxist intellectual Ralph Miliband, it is unlikely that Mr Miliband will disappear from the fray altogether, and his name will certainly be linked with any major international job which becomes vacant in the coming months.
Already, he is being mentioned in connection with the post of managing director of the International Monetary Fund, if current head Dominique Strauss-Kahn steps down as expected to run against Nicolas Sarkozy in the 2012 French Presidential elections. This could plunge him into yet another internecine battle, as Gordon Brown is also believed to be interested in the job.
And a second tilt at the Labour leadership cannot be entirely ruled out. Mr Miliband remains MP for South Shields and, at the age of just 45, he could be in contention to succeed his brother in five years' time if Ed fails in his ambition to return Labour to power - or even earlier if the party suffers a catastrophic plunge in the polls.
Wags in Manchester were today suggesting that David may be hoping to be the Michael Howard to Ed's Iain Duncan Smith - an older hand who steps in to restore the party's fortunes after the removal of an inappropriate and short-lived leader.
Throughout the leadership campaign, Ed made clear that he would be happy to take a shadow cabinet position if he did not win, while David was much more circumspect about taking a junior role to his younger brother.
Today's decision confirms the bitterness of the blow of being outstripped by his sibling in the race which he had prepared all his adult life to win.
Four years his junior, Ed followed David into a philosophy, politics and economics degree at Oxford University, into the Labour Party, into a senior backroom job as adviser to Tony Blair's government, into the House of Commons as an MP and then into the Cabinet.
But it was always David who was viewed as the fastest-rising star with the potential to become leader. He was the first to announce his candidacy when Mr Brown stood down in May, remained favourite until 24 hours before the results were announced and led the field through three rounds of vote-counting until finally being pipped by a margin of just 1.3%.
Observers suggested that he may have lost out because he did not have as much desire as his brother for the job, and was unwilling to make the concessions on Iraq, the economy and the unions needed to woo a party increasingly disillusioned with New Labour.
David was a close ally of Tony Blair from the earliest days of New Labour, working for him in opposition from 1994 and heading the No 10 policy unit during his first term in power.
Even before he became an MP, he was a key figure in reshaping the party's agenda, nicknamed "Brains" by Alastair Campbell for his powerful intellect and mastery of policy detail.
Parachuted into the safe seat of South Shields shortly before the 2001 election, he was a minister within a year and joined the Cabinet in 2005, rising to become environment secretary, where he put the issue of climate change firmly on the agenda for the first time.
When his former mentor quit 10 Downing Street in 2007, Mr Miliband consulted with Mr Blair over whether he should challenge Brown for the succession, but eventually decided to hold back from what would have been a bruising - and possibly fruitless - battle.
His appointment as the youngest foreign secretary for 30 years reflected Mr Brown's determination to mend fences with the Blairites and include talent from all sides of the party in his administration.
But doubts over his commitment to Brown were sparked by an article in 2008 in which he discussed the future of Labour without once mentioning the prime minister, which was widely perceived as disloyal and triggered calls for his dismissal.
All eyes were on him at the subsequent conference in Manchester, where he appeared to revel in the attention, memorably posing for pictures with a banana.
But he did not mount a challenge, using his conference speech instead to hail Mr Brown's leadership - the first of several occasions when he has been accused of "bottling it".
When his close friend James Purnell quit the Cabinet in June 2009 and when Patricia Hewitt and Geoff Hoon attempted a putsch in January this year, he held back from a resignation which could have given devastating momentum to their rebellions.
He may now be ruing those missed opportunities to seize a crown which seems to have slipped from his grasp forever.