Gen David Petraeus, US commander of Nato troops in Afghanistan, has handed responsibility for the military campaign there to his successor, Gen John Allen.Gen Petraeus, who is returning to the US to run the CIA, oversaw last year’s surge of US troops to Afghanistan.The transition comes after a spate of attacks against key figures in the Karzai administration as Nato forces start handing over control to Afghan forces.Sanjoy Majumder reports from Kabul.