From the point of view of cost and benefit, terrorism is a brilliantly effective form of warfare, especially against heavily armed countries of the First World. The reasons are several. First, terrorism offers no target to the basically World War Two militaries of advanced countries. If five Saudis, two Pakis, a Russian and a disaffected American blow up a building in Chicago, against whom does the US seek revenge? Is it against Russia, Pakistan, Saudi Arabia and the United States, none of whose governments had anything to do with the attack?