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.At the change of command ceremony in the Afghan capital, Kabul, Gen Petraeus said an "important juncture" had been reached in the joint campaign while Gen John Allen said he would "maintain the momentum" of the campaign.