UK forces in southern Afghanistan were under-resourced until extra foreign troops arrived earlier this year, the spokesman for the head of the Armed Forces has told an MPs' committee.
Major General Gordon Messenger said his own brigade had been "stretched" and unable to venture into some areas.
One MP said ministers had repeatedly insisted commanders were given the equipment and manpower they requested.
The US decided last December to boost its troop numbers by 30,000.
The extra soldiers were deployed to target the insurgency, help secure key population centres and bolster training of Afghan forces.
Announcing the surge, President Barack Obama said that existing US troops in the country lacked the full support they needed to effectively achieve these aims.
The deployment came from "the realisation that the scale of the challenge was not matched by the resources allocated to it", Maj Gen Messenger told the Commons Defence Select Committee.
The MPs questioned why it had taken so long for the Nato coalition to work out that more troops were needed.
Maj Gen Messenger, strategic communications officer to the chief of the defence staff, said only now had adequate resources been in place for long enough to be making a positive impact.
"In places like Helmand, which is as bad as it gets in terms of security across Afghanistan, we've started seeing far more positive indicators."
He said that when he returned from Afghanistan, where he served as a brigade commander until April last year, resources were inadequate.
"The obvious point at that time was that there were insufficient resources being allocated to the challenge in southern Afghanistan," he told the committee.
"I commanded a brigade, alongside an Afghan brigade commander, that was stretched and was not able to go to certain key areas where we knew we would ultimately have to go in order to secure the population."
He said there had been a subsequent "enormous inflow" of US troops, soldiers from other Nato nations and a huge rise in the number of Afghan National Army and Afghan National Police that were providing security.
"The picture now is a situation where the key areas of population in Helmand are now secured."
Following Maj Gen Messenger's remarks, Liberal Democrat MP Mike Hancock said: "We've been told by successive secretaries of state that when commanders asked for it they got what they wanted, whether that was equipment or manpower.
"If that's true what you are saying I can't understand why it took you so long to realise that we were badly off personnel-wise and that the only way we could deal with it was by having an enormous surge by the Americans."
Committee chairman James Arbuthnot, a Conservative MP, told Maj Gen Messenger: "You are an immensely reassuring man... but it would be more reassuring if you told us 18 months ago that we were getting things wrong, but I can't remember your doing so."
The number of UK troops killed in Afghanistan since 2001, is 341.