The enemy within (Economist.com)

Two related questions haunt Britain, and Europe, in the wake of the London attacks. First, what is it that prompts a small minority of the continent’s Muslims to shift from discontent or personal frustration to active terror? And second, was the attack on London indeed an act of home-grown terror, or an atrocity initiated by people in some distant war zone who had a grudge against Britain? The Leeds arrests, while impressively swift and a credit to the police, have proved there is no easy answer to these questions. In an age of globalised ideologies, globalised communications and porous borders, there is no real distinction between domestic and foreign threats.

The enemy within--Economist.com