There have been literally hundreds of attempts to define the word usefully; not wanting to be left out, let’s attempt our own. In considering war, looking to German language sources for insight is often productive. In this case, we can look back to a word from World War I: schrecklichkeit, literally “horribleness.” During the invasion of Belgium, in the face of civilian resistance, the invading army of von Moltke took brutal action against the Belgians: burning churches, libraries, homes; executing priests, resisters, and sometimes whole villages as an example to others. The stated aim was to inflict such brutal damage that the civilian population would be demoralized at relatively little military cost. I think this is the key to understanding terrorism, and so we will define it:

Terrorism is military action against a civilian population, whether by regular or irregular forces, intended to maximize the demoralizing effect of the action while minimizing the military force required.

The use of terror is generally understood as an unconventional tactic, although hardly an unknown one in conventional warfare, as the example of von Moltke shows. But then ours is an unconventional opponent: rather than a single state or alliance of states, our opponents are factions with some ideological commonalities, composed of small groups and even individuals of like mind but often of diverse intentions. This makes warfare against them much more difficult. Conventional warfare is effective as a rudder to turn the ship of state, but what we’re fighting is a flotilla of dinghies.

One objection to this definition might be that it includes too much: after all, the bombings of Dresden, Hiroshima, and Nagasaki were primarily intended not to destroy some military objective, but to break the will of the opponent. By this definition, these are themselves acts of terror. In my view, this criticism is both correct and pointless. Most all tactics of war have both an effect on the military forces and on the morale of the people involved. We choose, in general, to minimize the direct effects on civilians from military strikes, and it’s a point of pride for the American military that we try to minimize collateral damage. But the use of terror as a tactic must been seen in itself, as an attack on morale as well as people and assets. This definition, as offered, has this much to recommend it: it clarifies in the definition the specific, pragmatic military reasons for using terror as a tactic. Terror is used to break the will of an opponent at the least military cost.

It is a historical fact that the United States has never suffered a military defeat on military terms. It follows as a consequence that our opponents necessarily find themselves compelled to make war against us by unconventional means. It is also an unfortunate historical fact that the United States lost the war in Vietnam, arguably lost the war in Korea, and may right now be losing the campaigns in Iraq and Afghanistan because our various opponents have effectively fought us not through direct military action but by attacking our morale through unconventional means.

We call some of these means “terrorism,” but by using the word — and more so by treating the word as describing a crime rather than a kind of warfare — we protect our tender sensibilities from the realization that our opponents are, in an organized and concerted fashion, using force to bend us to their will. By definition, and by their intention, our opponents are in fact making war on us:

Der Krieg ist eine bloße Fortsetzung der Politik mit anderen mitteln. — Clausewitz

“War is merely a continuation of policy by other means” is a more commonly seen quotation from von Clausewitz, often mistranslated as “politics” or paraphrased as “diplomacy.” But in English, “politics” is the process through which policy is defined. The policy is the course of action proposed or to be taken. To von Clausewitz, war is a means by which policy is implemented. In other words: war is not a thing in itself; it is a tool through which an opponent is bent to our will, as expressed in our policy. To make war on terror, then, is to use force to achieve a policy end.

So what was our policy? Simply stated, the real problem of 9/11 is that our opponents were using “terrorism” as a tactic of war to bend the United States to their will. They used force, murdering thousands and attacking iconic buildings, to break our morale and to bring the United States to its knees. And our policy was to make sure they stopped doing that.