"Guerrilla warfare is a form of irregular warfare in which small groups of combatants, such as paramilitary personnel, armed civilians, or irregulars, use military tactics including ambushes, sabotage, raids, petty warfare, hit-and-run tactics, and mobility, to fight a larger and less-mobile traditional military. Although the term ‘guerrilla warfare’ was coined in the context of the Peninsular War in the 19th century, the tactical methods of guerrilla warfare have long been in use. In the 6th century BC, Sun Tzu proposed the use of guerrilla-style tactics in The Art of War. ...”
Guerrilla warfare during the Peninsular War, by Roque Gameiro, depicting a Portuguese guerrilla ambush against French forces. The term "guerrilla" was coined during this conflict, which occurred in the early 19th century.