NEW : NATO says a WikiLeaks reference to a withdrawal of troops in the past is because Afghan President Karzai expresses his gratitude for Britain 's support . `` It 's challenging , but it is achievable , '' Cameron said . British troops could start heading home by the end of 2011 . NATO 's goal is to turn over security operations in Afghanistan before 2015 .
NEW : NATO says a WikiLeaks reference to a withdrawal of troops in the past is because Afghan President Karzai expresses his gratitude for Britain 's support . British troops could start heading home by the end of 2011 . NATO 's goal is to turn over security operations in Afghanistan before 2015 .