Declining public support may harm the UK military mission in Afghanistan, the head of the armed forces has warned.
Air Chief Marshal Sir Jock Stirrup said the loss of backing at home was more damaging to the morale of troops than the weapons being used against them.
In his annual lecture to the Royal United Services Institute in London, the Chief of the Defence Staff said the Taliban could not win in Afghanistan.
But the international coalition could fail if it lost its nerve, he warned.
He said: "It's time we recalled those famous words 'in war, resolution'.
"This endeavour is important enough to our national security to justify the price our people are paying.
"The mission is achievable and at last we have a properly resourced plan to deliver the strategy."
"Our people in theatre know this. The greatest threat to their morale is not the Taliban or IEDs [improvised explosive devices], but declining will at home.
ACM Stirrup added: "Support for our service men and women is indivisible from support for this mission.
"Our people know that they can succeed, that we'll only fail if we choose to fail. We owe it to them, and to those we've lost, not to make that choice."
He said the Taliban were "war-weary" and "squabbling between themselves".
And the leadership of al-Qaeda had "suffered considerably" as a result of the campaign against it.
But he warned: "They're not finished. They could certainly recover if we reduced the pressure now.
"But conversely, if we are able to sustain that pressure for a while longer we may very well cause them irreversible damage."
Earlier this week the spokesman for British forces in Afghanistan, Lt Col David Wakefield, said there was a risk that troop morale could be damaged if public attitudes at home turned against the conflict.
He said: "The Taliban is not going to defeat us militarily, but we want the same patience, courage and discipline that soldiers show here from the public at home."