Mean Mr. Mustard makes some good points re: London:

This point is only more true however in the modern terrorism context. We simply can’t protect soft targets like subways and buses in a free society. We certainly can’t resort to a strategy of urban super-security that Israel subscribed to for the last decade. Our size doesn’t permit it (and it should be noted that even with Israel’s advantage in a smaller and more compact size didn’t mean the strategy was workable).

Quite simply, there are jackals prowling about outside. We can grab our guns and go out into the wilderness to hunt the ugly, treacherous animals down, or we can hide behind fences all day and night and even then accept that periodically there will be children snatched away when the opportunity arises.