What is the difference between insurgency and terrorism?

Terrorism is not an end in itself. Terrorism can be a subset of an insurgency, or it can be an act of violence by criminals or crazies who want to strike out at anyone especially a government. Timothy McVeigh committed a terrorist act in Oklahoma City but he was not part of any insurgency."

Doctrinally, we (DoD) define insurgency as 'an organized resistance movement that uses subversion, sabotage, and armed conflict to achieve its aims. Insurgencies normally seek to overthrow the existing social order and reallocate power within the country. They may also seek to (1) Overthrow an established government without a follow-on social revolution. (2) Establish an autonomous national territory within the borders of a state. (3) Cause the withdrawal of an occupying power. (4) Extract political concessions that are unattainable through less violent means.'

Doctrinally, guerrillas are the 'overt military aspect of the insurgency.' They exist alongside their counterparts, the auxiliary and the underground.

We find critical distinctions in these definitions. First, doctrine correctly identifies guerrillas as a subcomponent of insurgencies that work overtly toward the latter's counter-regime goals, typically organized not too unlike general purpose forces. Second, each of the five goals of an insurgency - the violent arm of a given resistance movement - centers on attacking regimes. In comparison, the goals of terrorists are not specific to governments but rather focus on broader ideological intentions. Furthermore, we see that terrorists may not even feel the need to target governments. Instead they may choose to attack societies directly in order to achieve a particular endstate. Hence, by definition terrorists are not concerned with regime change, reallocation of power, or challenging existing social orders.

Another way to look at it is this: insurgents use ideology to target governments, but terrorists target governments (or societies) to advance ideology.

Notwithstanding the differences between these irregular warriors, it is conceivable that a terrorist may also simultaneously be an insurgent and a guerrilla. Depending upon the ideology that the terrorist wants to advance, regime change may be a critical component of that effort. Marxist terrorists operating in capitalist or monarchial societies are good examples of ideologically-motivated terrorists who envision regime change as an integral component to their strategy."

… insurgencies combine violence with political programs in pursuit of revolutionary purposes in a way that terrorism cannot duplicate. Terrorists may pursue political, even revolutionary, goals, but their violence replaces rather than complements a political program.

If definitions offer only a partial aid in discriminating between terrorism and insurgency, organizational traits have traditionally provided another means to tell the two apart. Insurgencies normally field fighting forces orders of magnitude larger than those of terrorist organizations. Typically insurgents organize their forces in military fashion as squads, platoons, and companies. Terrorist units are usually smaller and comprised of isolated teams not organized into a formal military chain of command. Insurgent forces are often more overt in nature as well, especially in the sanctuaries or zones, which they dominate. Terrorist organizations, which tend towards extreme secrecy and compartmented cells to facilitate security, seldom replicate an insurgency's political structure."