Fred Reed writes at LewRockwell.com:
In recent years, I have seen terrorism denounced as a despicable crime. I wonder whether it shouldn’t be accepted frankly as a form of war. I am not sure why blowing up ten people in a restaurant in, say, London is more despicable than blowing up ten children in Afghanistan by a drone. (They are both despicable.) Some terrorists, such as the Unabomber, are merely freelance criminal psychopaths. Others, such as bin Laden, engage in terrorism for the same reason why militaries attack countries: to make the other side do what the attacker wants.
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?