Speaking in Edinburgh at the NATO parliamentary assembly, David Miliband has said that British troops will not remain in Afghanistan forever.
But the foreign secretary said that NATO could not risk withdrawing too soon, leaving behind it a power vacuum that the Taliban would fill.
His remarks build on Gordon Brown's foreign policy speech on Monday where he set out plans for an international conference on Afghanistan to facilitate an exit strategy for British forces.
Miliband told the assembled parliamentarians that Afghans feared that the West would tire of the war and that the Taliban will return.
And he said success would only come if the military strategy was backed up with a coherent political strategy.
He said that the insurgency was not a unified force, and NATO needed to show Taliban fighters that there was a "coherent and clear alternative" to fighting.
"Some Afghan Taliban may be committed to global jihad, the vast majority are not," he said.
"Our goal is not a fight to the death; it is to demonstrate clearly that they can not win."
He added: "This is not a war without end,"
The strategy depends on an Afghan government able to "act decisively" in the interests of the country.
"In the eyes of the afghan people and the wider world this means addressing the corrosive fear of corruption," he said.
Afghan president Hamid Karzai is due to be inaugurated later this week following elections that were blighted by widespread electoral fraud.
On the day that the MoD named the latest British serviceman to have been killed on operations there, Miliband acknowledged the heavy price being paid by UK forces in the region.
"We have suffered the bloodiest war since the Falkland's war," he said.
Rifleman Andrew Fentiman of 7th Battalion the Rifles is the 234th UK serviceman to have been killed in Afghanistan since operations began in 2001, 97 have died this year.
Speaking before the foreign secretary, NATO secretary general Anders Rasmussen called on members of the alliance to commit more troops to the mission.
He said: "To my mind it is obvious - that if we were to walk away and turn our backs on Afghanistan, al Qaida would be back in a flash.
"They would have a sanctuary from which to launch their strategy of global jihad, a strategy that is directed first and foremost against us.
"There's absolutely no reason to think otherwise and anyone who does so is not living in the real world."
