The term, the diminutive form of "war" in Spanish, is usually translated as "little war", and the word, guerrilla (Spanish pronunciation: [ɡeˈriʎa]), has been used to refer to the concept since the 18th century, and perhaps earlier. In correct Spanish usage, a person who is a member of a guerrilla is a guerrillero ([ɡeriˈʎeɾo]) if male, or a guerrillera if female. This term became popular during the early-19th century Peninsular War, when the Spanish people rose against the Napoleonic troops and fought against a highly superior army using the guerrilla strategy.
The term "guerrilla" was used in English as early as 1809, to refer to the fighters (e.g., "The town was taken by the guerrillas"), and also (as in Spanish) to denote a group or band of such fighters. However, in most languages guerrilla still denotes the specific style of warfare. 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.
"Guerrillas" usually carries positive connotations, and is often used by such fighters themselves and by their sympathizers, while their foes in many cases call them "Terrorists". Making an objective definition of the difference between "a guerrilla" and "a terrorist" has proven a difficult task.
The strategy and tactics of guerrilla warfare tend to focus around the use of a small, mobile force competing against a larger, more unwieldy one. The Guerrilla focuses on organizing in small units, depending on the support of the local population, as well as taking advantage of terrain more accommodating of small units.
Tactically, the guerrilla army would avoid any confrontation with large units of enemy troops, but seek and eliminate small groups of soldiers to minimize losses and exhaust the opposing force. Not limiting their targets to personnel, enemy resources are also preferred targets. All of that is to weaken the enemy's strength, to cause the enemy eventually to be unable to prosecute the war any longer, and to force the enemy to withdraw.
It is often misunderstood that guerrilla warfare must involve disguising as civilians to cause enemy troops to fail in telling friend from foe. However, this is not a primary feature of a guerrilla war. This type of war can be practiced anywhere there are places for combatants to cover themselves and where such advantage cannot be made use of by a larger and more conventional force.
Communist leaders like Mao Zedong and North Vietnamese Ho Chi Minh both implemented guerrilla warfare giving it a theoretical frame which served as a model for similar strategies elsewhere, such as the Cuban "foco" theory and the anti-Soviet Mujahadeen in Afghanistan. Mao Zedong summarized basic guerrilla tactics at the beginning of the Chinese "Second Revolutionary Civil War" as: "The enemy advances, we retreat; the enemy camps, we harass; the enemy tires, we attack; the enemy retreats, we pursue." At least one author credits the ancient Chinese work The Art of War (dating from at least 200 BC) with providing instruction in such tactics to Mao.
While the tactics of modern guerrilla warfare originate in the 20th century, irregular warfare, using elements later characteristic of modern guerrilla warfare, has existed throughout the battles of many ancient civilizations but in a smaller scale. This recent growth was inspired in part by theoretical works on guerrilla warfare, starting with the Manual de Guerra de Guerrillas by Matías Ramón Mella written in the 19th century and, more recently, Mao Zedong's On Guerrilla Warfare, Che Guevara's Guerrilla Warfare and Lenin's text of the same name, all written after the successful revolutions carried by them in China, Cuba and Russia respectively. Those texts characterized the tactic of guerrilla warfare as, according to Che Guevara's text, being "used by the side which is supported by a majority but which possesses a much smaller number of arms for use in defense against oppression".
The Chinese general and strategist Sun Tzu, in his The Art of War (6th century BC) or 600 BC to 501 BC, was the earliest to propose the use of guerrilla warfare. This directly inspired the development of modern guerrilla warfare. Guerrilla tactics were presumably employed by prehistoric tribal warriors against enemy tribes. Evidence of conventional warfare, on the other hand, did not emerge until 3100 BC in Egypt and Mesopotamia. Since the Enlightenment, ideologies such as nationalism, liberalism, socialism, and religious fundamentalism have played an important role in shaping insurgencies and guerrilla warfare.
One of the most remarkable guerrilla warfare warriors was Viriatus, a Lusitanian who led the resistance against the Roman Empire by obtaining several victories between 147 BC and 139 BC in the region of Zamora, Spain. Because of the innovative tactics he used during his command, he made himself the name of Terror Romanorum (Terror of the Romans).
A counter-insurgency or counterinsurgency (COIN) operation involves actions taken by the recognized government of a nation to contain or quell an insurgency taken up against it. In the main, the insurgents seek to destroy or erase the political authority of the defending authorities in a population they seek to control, and the counter-insurgent forces seek to protect that authority and reduce or eliminate the supplanting authority of the insurgents. Counter-insurgency operations are common during war, occupation and armed rebellions. Counter-insurgency may be armed suppression of a rebellion, coupled with tactics such as divide and rule designed to fracture the links between the insurgency and the population in which the insurgents move. Because it may be difficult or impossible to distinguish between an insurgent, a supporter of an insurgency who is a non-combatant, and entirely uninvolved members of the population, counter-insurgency operations have often rested on a confused, relativistic, or otherwise situational distinction between insurgents and non-combatants.
Theorists of counter-insurgency warfare have written extensively on the subject since the 1950s and 1960s but as early as the 1720s the third Marques of Santa Cruz de Marcenado (1684–1732) wrote that insurgencies were often the result of state failure and that the goal of those fighting the insurgents should be to seek the people's "heart and love". The two most influential of scholars of counter-insurgency have been Westerners whose job it had been to fight insurgents (often colonized people). Robert Thompson fought during the Malayan Emergency and David Galula fought during the Algerian War. Together these officers advocated multi-pronged strategies to win over the civilian population to the side of the counter-insurgent.
The widely distributed and influential work of Sir Robert Thompson, counter-insurgency expert of the Malayan Emergency, offers several such guidelines. Thompson's underlying assumption was that the counter-insurgent was committed to improving the rule of law and bettering local governance. Some governments, however, give such considerations short shrift. These governments are not interested in state-building and in extreme cases they have carried out counter-insurgency operations by using mass murder, genocide, terror, torture and execution. Historian Timothy Snyder has written, "In the guise of anti-partisan actions, the Germans killed perhaps three quarters of a million people, about 350,000 in Belarus alone, and lower but comparable numbers in Poland and Yugoslavia. The Germans killed more than a hundred thousand Poles when suppressing the Warsaw Uprising of 1944."
In the Vietnam War, the Americans "defoliated countless trees in areas where the communist North Vietnamese troops hid supply lines and conducted guerrilla warfare", (see Operation Ranch Hand). In the Soviet war in Afghanistan, the Soviets countered the U.S.–backed Mujahideen with a 'Scorched Earth' policy, driving over one third of the Afghan population into exile (over 5 million people), and carrying out widespread destruction of villages, granaries, crops, herds and irrigation systems, including the deadly and widespread mining of fields and pastures.
Some writers on counter-insurgency warfare emphasize the more turbulent nature of today's guerrilla warfare environment, where the clear political goals, parties and structures of such places as Vietnam, Malaysia, and El Salvador are not as prevalent. These writers point to numerous guerrilla conflicts that center around religious, ethnic or even criminal enterprise themes, and that do not lend themselves to the classic "national liberation" template.
The wide availability of the Internet has also caused changes in the tempo and mode of guerrilla operations in such areas as coordination of strikes, leveraging of financing, recruitment, and media manipulation. While the classic guidelines still apply, today's anti-guerrilla forces need to accept a more disruptive, disorderly and ambiguous mode of operation. According to David Kilcullen:
Insurgents may not be seeking to overthrow the state, may have no coherent strategy or may pursue a faith-based approach difficult to counter with traditional methods. There may be numerous competing insurgencies in one theater, meaning that the counterinsurgent must control the overall environment rather than defeat a specific enemy. The actions of individuals and the propaganda effect of a subjective "single narrative" may far outweigh practical progress, rendering counterinsurgency even more non-linear and unpredictable than before. The counterinsurgent, not the insurgent, may initiate the conflict and represent the forces of revolutionary change. The economic relationship between insurgent and population may be diametrically opposed to classical theory. And insurgent tactics, based on exploiting the propaganda effects of urban bombing, may invalidate some classical tactics and render others, like patrolling, counterproductive under some circumstances. Thus, field evidence suggests, classical theory is necessary but not sufficient for success against contemporary insurgencies.
Why does the guerrilla fighter fight? We must come to the inevitable conclusion that the guerrilla fighter is a social reformer, that he takes up arms responding to the angry protest of the people against their oppressors, and that he fights in order to change the social system that keeps all his unarmed brothers in ignominy and misery.
In the 1960s, the Marxist revolutionary Che Guevara developed the foco (Spanish: foquismo) theory of revolution in his book Guerrilla Warfare, based on his experiences during the 1959 Cuban Revolution. This theory was later formalized as "focalism" by Régis Debray. Its central principle is that vanguardism by cadres of small, fast-moving paramilitary groups can provide a focus for popular discontent against a sitting regime, and thereby lead a general insurrection. Although the original approach was to mobilize and launch attacks from rural areas, many foco ideas were adapted into urban guerrilla warfare movements.