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Well, according to the reports, Mr.Cameron defined the mission as this;
"You have to remember that Afghanistan is an extremely poor country with a very, very troubled history but I think the purpose of our mission was always to build an Afghanistan and Afghan security forces that were capable of maintaining a basic level of security so this country never again became a haven for terrorist training camps. That has been the most important part of the mission … The absolute driving part of the mission is the basic level of security so that it doesn't become a haven for terror.
"That is the mission, that was the mission and I think we will have accomplished that mission and so our troops can be very proud of what they have done."
But given the instances of blue on green attacks, given other reports of unreadiness of the Afghanistan army, and given the poor governance of Afghanistan, claiming that Afghanistan will never again become a haven for terrorist training camps seems slightly over-confident to me. I can see he might be very keen to acknowledge the work done by the British forces serving there, and to boost morale at Christmas and all that, but it still seems the kind of statement that can come around and bite you in the ass