I am not convinced that the US's presence in Afghanistan has a decisive effect on the incidence of terrorism outside Afghanistan. Thus I would have thought that the terrorism argument does not hold up so the US should go home where they belong (as many or more deaths per year through violence at home compared with terrorism elsewhere combined ?). On the other hand, for the US, UK and others, Afghanistan is almost certainly a very good and thoroughly realistic training ground for the military and (I can easily believe) a superb testing facility for new and existing hardware and technology. Think of it: Real-life scenarios, almost completely beyond any scrutiny.......