Well you can hide a knife down your pants until the very last minute. You can walk into a crowded airport carrying several gallons of petrol. Armed troops are useless against threats of that kind. I find the presence of police officers on the streets very comforting. And a good deal of great work is being done in unravelling the network that may have been behind this week's tragedy. Who knows. Capturing and punishing perpetrators has always been the way to go.