"Those fucking cunts," one of my men muttered. There was plenty of agreement, some just calling them curses, a few derogatory comments which, in that moment, could be well and truly forgiven. We'd spent time in Afghanistan and Iraq fighting some of these fuckers. We were not the first target. Madrid had been bombed the previous year, the first sign the war was expanding. The loss of life there had caused a grip of fear across Europe.

It was July 7th, 2005. What we were witnessing on the TV chilled us to the core. The war we'd been fighting thousands of miles away had visited the streets of London. In...