Sir Richard said troops had not been deployed as effectively as possible
The British Army was effectively providing "no security at all" in the southern Iraqi city of Basra by mid-2006, a senior soldier has said.
Lieutenant General Sir Richard Shirreff said that 200 troops were attempting to control a city of 1.3 million people, with militias "filling the gap".
He told the Iraq Inquiry the British Army had reached "stalemate".
It has also emerged that Jack Straw and Geoff Hoon will be the first members of Tony Blair's cabinet to give evidence.
Mr Hoon, who was defence secretary at the time of the invasion in 2003, will appear for two three-hour sessions on Tuesday 19 January.
Justice Secretary Mr Straw, who was foreign secretary at the time, will give evidence in a single three-hour session on 21 January.
During Monday's hearing, the panel heard from Sir Richard, who was commanding officer of the multi-national division in south-east Iraq from July 2006 until Jan 2007.
On Basra, he told the inquiry: "What I found when I arrived was effectively no security at all."
Sir Richard said that, for a city of 1.3 million people, only 200 troops could be sent out on patrol at one time.
AT THE INQUIRY
BBC World Affairs correspondent Peter Biles
Again, the length of military postings has been discussed at the Inquiry. The chairman, Sir John Chilcot, has frequently stressed that this is "a lessons learnt exercise".
So he asked Lt Gen Sir Richard Shirreff whether he thought a six-month tour of iraq was too short. Sir Richard was clear that senior commanders must do more than six months.
But he added: "For the soldiers, six months is a hell of a long time. It's hard, dangerous fighting. And in those temperatures, six months is about right."
Having heard evidence from the military top brass and from dozens of senior Whitehall officials in the weeks since the public hearings began in November, there will be a distinct change of tone on Tuesday.
Tony Blair's former spin-doctor, Alastair Campbell, will be on the witness stand. The reputation of Sir John Chilcot and his co-inquisitors will be under closer scrutiny than ever.
He said: "There was a significant lack of troops on the ground."
Once British forces moved on, "the militia filled the gap and effectively the militia controlled the city", he added.
Sir Richard said: "It was not what I expected. It was clear to me that the intention, from a British perspective, was to progress to Iraqi control as far as possible.
"Equally, it was clear to me that there could be no transition to Iraqi control without security."
Sir Richard organised more training of Iraqi forces during his time in charge, he told the inquiry.
Beforehand, he said: "I don't think the [UK's] troops were deployed as effectively as the needed to be deployed... Troops that could have been used on the ground perhaps were tied up with guarding security convoys...
"What we had reached was a position of stalemate and the momentum was going downhill...
"It was pretty clear to me... that we had a strategy that involved extraction rather than achieving mission success. It was about an exit strategy rather than a winning strategy... A winning strategy was going to require significant extra resources."
Sir Richard was critical of the level of equipment available to the mission in the south of Iraq.
He said: "We had no UAV [unmanned aerial vehicles] capable of flying over Basra."
He added: "I was told that no more [resources] could be used to putting UAVs into southern Iraq... I think the Ministry of Defence was incapable of creating the drive."
Sir Richard said defence spending cuts in 2003/04 had meant a cut in the Army's infantry numbers.
Describing his efforts to procure greater resources, he said: "I got what was possible, as far as possible... But I would stress, though, that what was possible was not going to be enough."
On troop deployment, Sir Richard said: "Clearly the risk was that, once we moved away from one area [of Basra], we would lose it to the militia..."
He said: "'Good enough' was the motto, not 'perfection'."
Sir Richard spoke of a "a difference of opinion" between himself and London, adding that he felt "the gravity of the situation was not fully appreciated. The focus was on exit rather than achieving relevant success".
But he also said: "I say again that there was a lot of hard work and a lot of hard work went into improving capability."
The Iraq Inquiry is looking into UK policy on the country between 2001 and 2009, when British troops left the country. A report is due to be published late this year or in early 2011.