The West can only contain, not defeat, militant groups such as al-Qaeda, the head of the UK's armed forces has said.
General Sir David Richards, a former Nato commander in Afghanistan, said Islamist militancy would pose a threat to the UK for at least 30 years.
But he told the BBC's Andrew Marr Show a clear-cut victory over militants was not achievable.
The BBC's Frank Gardner said the comments reflect a "new realism" in UK and US counter-terrorism circles.
Our security correspondent said such an admission five years ago might have been considered outrageous and defeatist.
Gen Richards, 58, took over as chief of the defence staff last month, after a spell as head of the British army.
'Secure lives'
Before he was due to lay a wreath at the Cenotaph as part of the UK's Remembrance Sunday commemorations, Gen Richards told the BBC's Andrew Marr Show it was important to get the balance of remembrance right.
"It's something we've got to be very careful about... there's a lot of dwelling on death as opposed to what those people who have died achieved in their sadly too often too brief lives, but those people have done immense things that are good and I think we need to focus a bit more on that," he said.
Britain has lost 343 soldiers in Afghanistan since 2001.
But Gen Richards told the BBC it was not possible to defeat the Taliban or al-Qaeda militarily.
"You can't. We've all said this. David Petraeus has said it, I've said it.
"The trick is the balance of things that you're doing and I say that the military are just about, you know, there.
"The biggest problem's been ensuring that the governance and all the development side can keep up with it within a time frame and these things take generations sometimes within a time frame that is acceptable to domestic, public and political opinion," he said.
