While having an unbiased definition of terrorism is the first step toward moral credibility, the second step is to base our condemnation of terrorism on the impartial application of a general moral principle. The simplest, most plausible moral principle for this purpose is one that forbids all intentional killings of innocent people. This is what the principle of noncombatant immunity does in the context of war and political violence.

The principle of noncombatant immunity is one half of the principle of discrimination. The first half is a permission that allows soldiers to attack enemy soldiers in war while the second prohibits soldiers from attacking civilians. Terrorism violates this prohibition by intentionally directing its attacks against civilians.

If noncombatant immunity is an absolute principle that permits no exceptions, then it will follow that terrorism is always wrong. The argument would be simple and direct.

1. 1. Terrorist acts always intentionally kill or injure innocent people.

2. 2. It is always wrong intentionally to kill or injure innocent people.

3. 3. Therefore, terrorist acts are always wrong.

In this argument, the view that terrorism is always wrong is not a necessary truth. It is an inference from the definition of terrorism and the moral principle that condemns all attacks on innocent people. Since the idea of intentionally killing innocent people often strikes people as morally repellent, we might think that statement 2, the principle of noncombatant immunity, needs no justification. In fact, however, this principle has been doubted, criticized, and rejected by many people.