In a global war on terrorism, it is important to ask what we mean by terrorism. The usual definition of terrorism is something like "the use or threat of violence, by small groups against non-combatants of large groups, for avowed political goals." The key to this definition is the combination of small groups killing non-combatants. Terrorism is the warfare of the weak, the recourse of those desperate for a cause that cannot win by conventional means. But it is worth noting that state terrorism against a state's own citizens--as practiced by Mao, Stalin, Hitler, Pol-Pot, and many smaller-league tyrants--has killed millions of non-combatants, whereas the anti-state terrorism we usually focus on has killed thousands.
McCauley, Clark R. 2001. "The Psychology of Terrorism." Social Science Research Council (October). http://essays.ssrc.org/sept11/essays/mccauley.htm