The head of Britain's Armed Forces has warned that the West will never be able to defeat al Qaida and Islamic militants.
General Sir David Richards said the national security of Britain is still at stake, but the threat can be contained to allow Britons to lead secure lives.
In an interview with The Sunday Telegraph, he also revealed that Prince William is unlikely to serve in Afghanistan but suggested Prince Harry could return to front-line duty in the campaign as a helicopter pilot.
The commander was speaking ahead of the Remembrance Sunday ceremony in Whitehall, where he will lay a wreath in memory of Britain's war dead.
Gen Richards, 58, told the paper: "Make no mistake, the global threat from al Qaida and its terrorist affiliates is an enduring one and one which, if we let it, will rear its head in states, particularly those that are unstable.
"The national security of the UK and our allies, is in my judgment, at stake."
He added: "In conventional war, defeat and victory is very clear cut and is symbolised by troops marching into another nation's capital. First of all you have to ask: do we need to defeat it (Islamist militancy) in the sense of a clear cut victory? I would argue that it is unnecessary and would never be achieved.
"But we can contain it to the point that our lives and our children's lives are led securely? I think we can."
The general said of the men and women fighting in Afghanistan: "I think there are direct parallels to be made with the bravery of those who risked, and who gave, their lives in the fight against fascism in the Second World War."
He also said the British military and the Government had been "guilty of not fully understanding what was at stake" in Afghanistan and admitted the Afghan people were beginning to "tire" of Nato's inability to deliver on its promises, but he insisted the sacrifice being made by British troops in Afghanistan, where 343 soldiers have been killed since 2001, "has been worth it".