Guerrilla warfare is the unconventional warfare and combat with which a small group of combatants use mobile tactics (ambushes, raids, etc.) to combat a larger and less mobile formal army. The guerrilla army uses ambush (draw enemy forces to terrain unsuited to them) and mobility (advantage and surprise) in attacking vulnerable targets in enemy territory.

This term means "little war" in Spanish and was created during the Peninsular War. The concept acknowledges a conflict between armed civilians against a powerful nation state army. This tactic was used by the Viet Cong and North Vietnam Army in the Vietnam War. Most factions of the Iraqi Insurgency and groups such as FARC are said to be engaged in some form of Guerrilla warfare.

Etymology

Guerrilla means small war, the diminutive of the Spanish word Guerra (war). The Spanish word derives from the Old High German word Werra and from the middle Dutch word warre; adopted by the Visigoths in A.D. 5th century Hispania. The use of the diminutive evokes the differences in number, scale, and scope between the guerrilla army and the formal, professional army of the state. The word was coined in Spain to describe their warfare in resisting Napoleon Bonaparte's French régime during the Peninsula War, its meaning was broadened to mean any similar-scale armed resistance. Guerrillero is the Spanish word for guerrilla fighter, while in Spanish-speaking countries the noun guerrilla usually denotes guerrilla army (e.g. la guerrilla de las FARC translates as "the FARC guerrilla group"). Moreover, per the OED, 'the guerrilla' was an English usage (as early as 1809), describing the fighters, not only their tactics (e.g."the town was taken by the guerrillas"), however, in most languages guerrilla still denotes the specific style of warfare. Fact: date=November 2007

Guerrilla warfare as a continuum

An insurgency, or what Mao Zedong referred to as a war of revolutionary nature, guerrilla warfare can be conceived of as part of a continuum. On the low end are small-scale raids, ambushes and attacks. In ancient times these actions were often associated with smaller tribal polities fighting a larger empire, as in the struggle of Rome against the Spanish tribes for over a century. In the modern era they continue with the operations of insurgent, revolutionary and "terrorist" groups. The upper end is composed of a fully integrated political-military strategy, comprising both large and small units, engaging in constantly shifting mobile warfare, both on the low-end "guerrilla" scale, and that of large, mobile formations with modern arms.

The latter phase came to fullest expression in the operations of Mao Zedong in China and Vo Nguyen Giap in Vietnam. In between are a large variety of situations - from the wars waged against Israel by Palestinian irregulars in the contemporary era, to Spanish and Portuguese irregulars operating with the conventional units of British General Wellington, during the Peninsular War against Napoleon.