The Defence Secretary Des Browne admitted today that the scale of the task in Afghanistan had been underestimated by Britain and Nato.
Mr Browne said the Government had been under no illusions that the operation would be tough, but it had proved "even harder" than predicted.
In a speech to the Royal United Services Institute in London, he said: "We always knew the south (of Afghanistan) would be hard.
"We do have to accept that it's been even harder than we expected.
"The Taliban's tenacity in the face of massive losses has been more than we expected."
Mr Browne said there had been "very little progress in governance and reconstruction" in the south of the country.
He said the lack of progress there was perhaps "inevitable at this stage", and things were getting better in other areas.
However, Mr Browne insisted the operation had enjoyed "broad support" internationally from the outset, and success was "essential" to global security.
"Success won't be what we understand to be security and prosperity and proper governance, but it will be progress and it will be massively worth it," he added.
He also rejected the idea that Nato forces were bound to fail in the same way as the Russians and British previously had in Afghanistan.
Mr Browne admitted Nato was "still learning" in Afghanistan, but urged commentators not to "second guess" the operational needs of commanders.
He challenged Nato nations to "reaffirm" their commitment to seeing the task through, conceding it had been difficult to raise extra troops.
"Nato have estimated they need 2,500 more troops, 1,000 of whom are combat troops. We are still in the middle of the process of finding them.
"There is no denying it has been difficult and that we are not there yet."
He said candidly that while nations should speak for themselves, "some have doubts that the mission can succeed" while others were concerned about "the level of risk" their soldiers would be exposed to.
But he stressed that the mission in Afghanistan remained as vital as it had ever been, not just to the region but to the wider world, and for Nato's own credibility.
"Nato nations must decide whether to back their investment, re-affirm their original intent and send a clear signal that Nato as an alliance is strong and determined to see the task through," said Mr Browne.
"All partners should be prepared to face equal risk. No-one has a monopoly on determination and courage."
He stressed that Nato's foes had been "fought to a standstill. They cannot beat us".
But that was only the first step, said the Defence Secretary.Reuse content