Before Delta became Delta, the SAS had already carved the rough shape of modern counterterrorism into the stone, and the Americans were smart enough to study the blade marks.

The sighting fits a pattern increasingly familiar across the UK, with urban terrain being used for secretive training missions.

In September 2005, British special-operations forces found themselves facing a problem. Two of their own had been captured and beaten by local Iraqi police during an undercover operation in Basra, and ...