The Archbishop of Canterbury has warned that the US and UK will be "held to account" over Iraq, at a remembrance service for those who died in the war.
Families of the 51 Britons killed were in attendance
The Queen and the prime minister joined relatives of the 51 British personnel killed in Iraq at the service at St Paul's Cathedral in central London.
In his address, Dr Rowan Williams urged coalition leaders to check that the "integrity" of their mission in Iraq was not under strain.
Before the service, some of the families who are angry at the way the war has been handled, called on Tony Blair to stay away.
The emphasis of the service was on remembrance rather than victory, and military pomp was kept to a minimum.
The service, attended by about 2,000 people, began with prayers for all those who were killed, British and Iraqi alike.
Then Dr Williams addressed the congregation, telling the 250 relatives present: "You know the cost [of war] in a unique way."
Dr Williams paid tribute to those who had lost their lives, and called for a reflection on the aims of the conflict, and on what has happened in Iraq since the conflict officially ended.
He said: "We have to go back and test what has happened in the light of the original vision; we have to find out what we have learned, what now looks different,
where our integrity has been stretched or challenged."
He reminded UK and US leaders they would be held accountable for what was happening in Iraq.
But he also condemned those who ignored world oppression without intervention.
The Bishop to the Forces, the Rt Rev David Conner, led prayers for peace and for "all who work in great danger day by day" in Iraq.
The Duke of Edinburgh and Air Chief Marshal Brian Burridge read
the lessons before a specially composed anthem sung by the cathedral choir.
The Bishop of London, the Right Rev Richard Chartres, led prayers for penitence
Roman Catholic Archbishop of Westminster Cardinal Cormac Murphy-O'Connor and
Methodist Conference vice-president Judy Jarvis called for "an end to
injustice, terrorism and war".
The traditional tune Last Post preceded two minutes' silence, broken by buglers sounding Reveille.
Before the service, some families voiced their disquiet at the government, including Defence Secretary Geoff Hoon, being so involved in the service.
Reg Keys, of north Wales, whose son Tom died alongside five other military policeman six weeks after the end of hostilities, was bitter that Mr Blair was attending the service.
"Let him see the misery that his decision to go to war has caused. Give the man a front seat but I won't sit next to him," he said.
However Peter Brierley said his son Shaun, who was killed in March, justified the war because Saddam Hussein had been removed.
After the service, he said it had been a
"Hopefully, there will have been
enough prayers said to get the peace that we really should
have around the world," he said.
The Queen as head of all the forces led the Royal Family
Royal Marines and Navy spokesman
Ben Curry said it had been a "calm and peaceful service".
"I lost a couple of good friends in an early helicopter crash
and it was a fitting service for them and for the nation," he said.
"It wasn't a thanksgiving service but it was important to say
thank you to all the services involved.
"It was an act of remembrance
and was very poignant. There were a lot of different emotions for
According to Ministry of Defence figures, 51 soldiers have died in Iraq - 21 killed in action, 19 in accidents and five in cases of so-called friendly fire.
A further four died of natural causes, while two deaths are still under investigation.
Unlike after previous conflicts, no victory parade or military march past has been held.