As war in Afghanistan persists and British soldiers continue to lose their lives fighting an insurgency that many commentators say they can't defeat, reporter Stephen Grey investigates whether our military is engaged in an impossible mission. In an unprecedented series of interviews, senior military commanders - including Air Chief Marshal Sir Jock Stirrup, chief of the defense staff, and General Sir Richard Dannatt, chief of the general staff - reveal what they think of the successes, failures and lessons learned during Britain's three-year intervention into Helmand province.