The strategy of irregular warfare has been used since ancient times, but the term “guerrilla warfare” seems to have originated in early-19th-century Spain during the Napoleonic wars. “Guerrilla” is the diminutive of the Spanish word for war—guerra. During the Napoleonic wars, British troops used the term guerrillero (warrior) to refer to the Spanish and Portuguese rebels. The form of irregular warfare waged by these resistance fighters, who were engaging French troops during the Napoleonic invasion and occupation, became known as “guerrilla warfare.” The term was then used to refer to rebel troops in the Americas who led the battles for independence against Spanish troops. More recently, “guerrilla warfare,” as both a strategy and an ideology, is most closely associated with the Cuban Revolution of 1959 and the subsequent publication of the treatise and manual Guerrilla Warfare by Ernesto “Che” Guevara. Guevara did not invent the idea of guerrilla warfare, but the unique (and ultimately successful) approach to making revolution in Cuba and Guevara’s important treatise on the subject did change the general understanding and meaning of the concept. Guevara’s explanation of guerrilla warfare in the context of the armed revolutionary struggle in Cuba changed the trajectory of Marxist revolutionary thought and actions in the 20th century as well.