UK officials wanted the UN to play a key role in post-war Iraq
There was a "dire" lack of planning in Washington for what would happen in the aftermath of the Iraq invasion, a senior British diplomat has said.
The US had a "touching faith" that its troops would be welcomed in Iraq and democracy would soon follow, Edward Chaplin told the Iraq inquiry.
UK officials defended their input into post-war planning but said they needed more resources to do the job better.
The inquiry is examining UK involvement in Iraq between 2001 and 2009.
The first two weeks of public hearings are focusing on the build-up to war, US-UK relations during the period and post-war planning.
Thousands of Iraqis died in sectarian violence in the months after the 2003 invasion as law and order broke down in many parts of the country.
Mr Chaplin, director of Middle East policy at the Foreign Office in the run-up to the 2003 invasion and ambassador to Baghdad after the war, said preparations for what would follow the toppling of Saddam Hussein were a "real blind spot" in Washington.
Although the State Department looked at the issue in detail, Mr Chaplin said there was less interest "elsewhere" in Washington and that policy was largely dictated by the White House and Pentagon.
From the BBC's Peter Biles at the Chilcot inquiry in London:
Today's witnesses, Sir Peter Ricketts and Edward Chaplin, have shed more light on the chaotic nature of the post-war planning.
The British government emerges with some credit for at least having started thinking about this in late 2002, some six months before the invasion of Iraq was launched.
The US administration however, stands accused of having had "a blind spot", and of having assumed that Iraqis would be grateful for the overthrow of Saddam Hussein. This optimism was not shared in London.
Then, with war looming, the Pentagon seized the lead role from the State Department, so undermining the planning work that had been done in Washington.
British views were listened to, but in Foreign Office jargon, the UK ideas did not get "traction". Britain was left playing second fiddle.
More will emerge in future sessions but the evidence has already given us some understanding of how and why Iraq went so badly wrong.
Foreign Office officials who raised the issue with their State Department counterparts had been urged to press home the point higher up in government, he said.
While US policymakers listened to UK ideas about post-war planning - and Tony Blair talked to President Bush about the issue - their input did not gain "much traction".
"They [US officials] had a touching faith that once Iraq had been liberated from the terrible tyranny of Saddam Hussein everybody would be grateful and dancing in the streets and there would be really be no further difficulty.
"And then the Iraqis would somehow magically take over and restore their state to the democratic state it should be in.
"We tried to point that this was extremely optimistic."
The US put too much faith in Iraqi opposition groups and political exiles about how quickly the country could be stabilised after the invasion, Mr Chaplin said, while coalition forces did not fully realise how "fractured" Iraqi society had become under Saddam Hussein.
Mr Chaplin said the UK tried to place senior officials in key positions in the Coalition Provisional Authority - which initially administered the country - to try to exert influence, but this was not "easy".
"We did our best to influence them [US officials] in what we thought was the right direction," he said.
Another senior Foreign Office official, Sir Peter Ricketts, said the UK began planning for the aftermath of a potential conflict in the autumn of 2002, focusing on giving the UN a key role in the process - something the US was adamantly opposed to.
Sir Peter, director general of the Foreign Office's political department between 2001 and 2003 and now head of the Diplomatic Service, said ministers believed that post-war planning should be taken "just as seriously" as the military campaign.
November-December: Former top civil servants, spy chiefs, diplomats and military commanders to give evidence
January-February 2010: Tony Blair, Gordon Brown and other politicians expected to appear before the panel
March 2010: Inquiry expected to adjourn ahead of the general election campaign
July-August 2010: Inquiry expected to resume
Report set to be published in late 2010 or early 2011
UK work focused on Iraq's likely humanitarian needs, he said, and political questions such as what elements, if any, of Saddam Hussein's regime could play a part in a transition.
But he said it soon became clear that US-led efforts to stabilise the country after the fall of Saddam were a "shambles".
Asked about the UK's post-war role in helping reconstruction around Basra in southern Iraq, Sir Peter said the objectives of the mission soon "outstripped" the money provided by the government.
"I think we could have done with more resources to back up the ambition to play an exemplary role," he said.
"There was an underestimate of the number of people and cost of the role we found ourselves playing in the south."
Post-war planning has already been heavily criticised with ex-British ambassador to the US, Sir Christopher Meyer, saying it was a "black hole" and claiming the UK did not exert enough influence over it.