A CIA library entry written by a Brit. I find it continually fascinating how close the US-UK intelligence and defence establishments are. Can anyone speculate

A CIA library entry written by a Brit. I find it continually fascinating how close the US-UK intelligence and defence establishments are. Can anyone speculate at the reasons for this? Is it the extreme cultural closeness, the Special Relationship fostered through WW2, or simply realpolitik -- i.e. US-UK having broadly speaking the same national interests? Or something else? Or do we just get on so well because we like each other's accents? :)