The strength of the Taliban in Afghanistan had been underestimated by Britain and Nato, the defence secretary has admitted.
Des Browne told the Royal United Services Institute (Rusi) that fighting the Taliban was "even harder than we expected", but insisted that achieving peace in the region was not "mission impossible".
The defence secretary promised "success" would be achieved, but said it would not "be what we understand by security and prosperity and proper governance, but it will be progress and it will be massively worth achieving".
"The Taliban's tenacity in the face of massive losses has been a surprise, absorbing more of our effort than we predicted it would and consequently slowing progress on reconstruction," he admitted.
Despite around 4,000 UK troops being based in Afghanistan's southern Helmand province, Mr Browne appealed to Nato to send more soldiers, claiming they must do so to prove they are "strong and determined to see the past through".
Defending Britain's strategy, he said the troops were there legitimately, with a "strong moral incentive" to "ensure the country does not slip back under Taliban" and "does not slide into civil war".
Defeating the Taliban was "as vital as it has always been", and the "mission" was "not only crucial for Afghanistan", but also for the threat the Taliban "poses to the world".
British people were all too aware, he argued, of the delivery of terrorism "on to the streets of the west".
Since the operations began in Afghanistan in 2001, 40 British soldiers have been killed - 19 of them this month.
Yesterday there were three separate suicide bombs leaving a total of 19 people dead.