The Guerilla was added in Civilization III: Play the World.
Guerillas are typically members of an irregular military force fighting small-scale, limited actions, in concert with an overall political-military strategy, against conventional military forces. Guerrilla tactics involve constantly shifting attack operations and include the use of sabotage and terrorism. Traditionally, guerrilla warfare has been a weapon of protest against alleged wrongs imposed on a people by a foreign invader or ruling government. Guerrillas may operate independently or complement orthodox military operations. The underlying strategy in guerrilla warfare is to harass the enemy until sufficient military strength has accumulated in order to defeat him in battle or until enough political and military pressure is applied to cause him to seek peace.