UK troops encountered less opposition than expected, the generals said
The man who led UK troops into Iraq in 2003 says he was told 10 months earlier that it was a matter of "when not if" the US would pursue military action.
Air Chief Marshal Sir Brian Burridge said General Tommy Franks, commander of US forces, told him in May 2002 that he hoped the UK would be "alongside".
He told the Iraq inquiry the campaign was conducted to minimise the impact on Iraqi civilians and key infrastructure.
The inquiry is examining UK policy towards Iraq between 2001 and 2009.
The first few weeks have focused on policy in the run-up to the war, the UK's assessment of Iraq's weapons capacity, military preparations for the invasion and post-war planning.
Sir Brian, who led UK forces on the ground in Iraq, said the possibility of military action was first raised with him in May 2002.
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
He recalled how Gen Franks had told him that "in terms of Iraq, it is not if but when" and that he had learned any offensive could happen in early 2003.
Although Gen Franks said he hoped the UK would contribute troops to any military operation, Sir Brian said this was never assumed.
"All participation in planning was without commitment," he said.
"It was absolutely clear that the UK had a view about the process that needed to be gone through."
As planning intensified, Sir Brian told the inquiry about the scenarios envisaged in the event of an invasion, ranging from the "immediate collapse" of Saddam Hussein's regime or its gradual disintegration to the possibility of a prolonged siege of Baghdad similar to "Stalingrad".
However, he said he had been prepared to stand the troops down at any stage if orders came from London and had not been convinced "until the last minute" that the invasion would go ahead.
The man who led UK forces in the south of the country, Lt Gen Robin Brims, said troops had encountered less opposition from the Iraqi army than anticipated, but more resistance than expected from irregular forces loyal to the regime.
He had been told by British diplomats not to "trash" Basra when UK forces took control of the city in early April. British troops were "reasonably well received" when they entered the city, he said.
But Gen Brims criticised subsequent reconstruction efforts there, blaming a lack of co-ordination and saying he did not recall the UK paying for things such as police salaries - relying instead on US funds.
Earlier, the man responsible for compiling a 2002 dossier claiming that Iraq would be able to deploy weapons of mass destruction "within 45 minutes", denied manipulating information about the Iraqi threat.
But Sir John Scarlett, ex-chairman of the Joint Intelligence Committee, said the 45-minute claim could have been "clearer" in what it referred to.
In its first few weeks, the Iraq Inquiry is hearing from senior diplomats and policy advisers who shaped policy in the run-up to the war.
The crucial question of the legality of the war will not be addressed until early next year, when Tony Blair is expected to give evidence.