World leaders are meeting in Lisbon today for a crucial Nato summit focusing on the war in Afghanistan and how to withdraw troops within the next four years. David Cameron and Barack Obama will sit down with Nato and the EU along with Afghan president Hamid Karzai and UN Secretary General Ban Ki-Moon. The allies appear to agree the target year 2014 is realistic but that does not mean the war is ending. The US is wary of giving the impression that the original aim of invading Afghanistan in 2001 - to deny al Qaeda a base to launch more terrorist attacks on the West - will be achieved by then. So Nato plans to pledge an enduring partnership with Afghanistan at the two-day Lisbon summit, while admitting past mistakes.I think that, seen retrospectively, we underestimated the challenge and our operation in Afghanistan didn't have sufficient resources, and yes, that was a mistake, Nato Secretary General Anders Fogh Rasmussen told Portugal's media. He added: We'll make a positive announcement in Lisbon that the handover is about to begin.