An insoluble problem. Not only is there the question of how far out you move the security perimiter and for how long the authorities can remain on high alert, but there is also the small matter of displacement.

The number of potential targets is endless, increase security at one and the others become all the more attractive. Trains, buses, concert halls, ferries, subways, you name it. The latter must be especially vulnerable and appealing to terrorists, given that narrow tunnels greatly enhance the blast effect of an explosion.

One could go on and on, there is no way that even a fraction of targets could be given anywhere near adequate protection. That puts the authorities in a bind, perhaps inducing them to resort to very visible measures that have very little impact.

Johan