I suspect that most of us who are old enough to do so remember where we were on 11 September 2001. I was sitting at my desk in London, wrestling with a book proposal, when a member of a military history email discussion list (do such things still exist?) mentioned that, from his office in New Jersey, he could see smoke rising from the Twin Towers. Apparently, he said, a light aircraft had crashed into them.

For the sixteen years since then, violent Islamist radicalism has overshadowed the foreign policies of Western democracies and, increasingly, domestic security policy as well. Western interventions in Afghanistan, Iraq, Libya and now Syria (and Iraq again) have expended blood and treasure to an enormous degree and are likely to continue