The head of British armed forces on Sunday warned that western forces would never be able to fully "defeat" al-Qaeda-inspired terrorism and could at best "contain" it.

In remarks that commentators described as reflecting "new realism" about the limits of using military power to defeat terrorism, General Sir David Richards said it was "unnecessary" even to think in terms of a "clear-cut victory’’ because it would "never be achieved".

"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 in the sense of a clear-cut victory? I would argue that it is unnecessary and would never be achieved. But can we contain it to the point that our lives and our children’s lives are led securely? I think we can," Gen Richards told The Sunday Telegraph.

He said Islamist extremism could not be completely eradicated and al-Qaeda and its affiliates would continue to pose a threat for at least 30 years.

"Make no mistake , the global threat from al-Qaeda 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," he said.

Gen Richards criticised British and American approach to the conflict in Afghanistan and said that Afghan people were starting to "tire’’ of NATO’s inability to deliver on its promises. Both Britain and America were sidetracked by Iraq and, initially, there were not enough troops in Afghanistan.

"To begin with our eyes were taken off the ball because of Iraq. I remember having a conversation about it with Donald Rumsfeld (the then US Defence Secretary). (It) got me into a bit of trouble for suggesting that the number of troops were not sufficient for the task. Today, everyone understands you need a certain amount of troops but there was a real debate about it then," he said.

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