Guerrillas
Etymology: The Spanish word guerrilla is the diminutive form of guerra ("war"); hence, "little war". The term became popular during the early-19th century Peninsular War, when, after the defeat of their regular armies, the Spanish and Portuguese people successfully rose against the Napoleonic troops and defeated a highly superior army using the guerrilla strategy in combination with a scorched earth policy and people's war (see also attrition warfare against Napoleon). In correct Spanish usage, a person who is a member of a guerrilla unit is a guerrillero ([geriˈʎeɾo]) if male, or a guerrillera ([geriˈʎeɾa]) if female. Arthur Wellesley adopted the term "guerrilla" into English from Spanish usage in 1809, to refer to the individual fighters (e.g., "I have recommended to set the Guerrillas to work"), and also (as in Spanish) to denote a group or band of such fighters. However, in most languages guerrilla still denotes a 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.

History: Prehistoric tribal warriors presumably employed guerrilla-style tactics against enemy tribes:

Primitive (and guerrilla) warfare consists of war stripped to its essentials: the murder of enemies; the theft or destruction of their sustenance, wealth, and essential resources; and the inducement in them of insecurity and terror. It conducts the basic business of war without recourse to ponderous formations or equipment, complicated maneuvers, strict chains of command, calculated strategies, timetables, or other civilized embellishments.
Evidence of conventional warfare, on the other hand, did not emerge until 3100 BC in Egypt and Mesopotamia. The Chinese general and strategist Sun Tzu, in his The Art of War (6th century BC), became one of the earliest to propose the use of guerrilla warfare. This inspired developments in modern guerrilla warfare.
In the 3rd century BC, Quintus Fabius Maximus Verrucosus, used elements of guerrilla warfare, such as the evasion of battle, the attempt to wear down the enemy, to attack small detachments in an ambush and devised the Fabian strategy, which the Roman Republic used to great effect against Hannibal's army, see also His Excellency : George Washington: the Fabian choice. The Roman general Quintus Sertorius is also noted for his skillful use of guerrilla warfare during his revolt against the Roman Senate.
In the medieval Roman Empire, guerrilla warfare was frequently practiced between the eighth through tenth centuries along the eastern frontier with the Umayyad and then Abbasid caliphates. Tactics involved a heavy emphasis on reconnaissance and intelligence, shadowing the enemy, evacuating threatened population centres, and attacking when the enemy dispersed to raid. In the later tenth century this form of warfare was codified in a military manual known by its later Latin name as De velitatione bellica ('On Skirmishing') so it would not be forgotten in the future.
The Normans often made many forays into Wales, where the Welsh used the mountainous region, which the Normans were unfamiliar with, to spring surprise attacks upon them.
Since the Enlightenment, ideologies such as nationalism, liberalism, socialism, and religious fundamentalism have played an important role in shaping insurgencies and guerrilla warfare.
In the 17th century, Chatrapati Shivaji Maharaj, founder of the Maratha Kingdom, pioneered the Shiva sutra or Ganimi Kava (Guerrilla Tactics) to defeat the many times larger and more powerful armies of the Mughal Empire.
Kerala Varma (Pazhassi Raja) (1753–1805) used guerrilla techniques chiefly centred in mountain forests in the Cotiote War against the British East India Company in India between 1793 and 1806. Arthur Wellesley (in India 1797–1805) had commanded forces assigned to defeat Pazhassi's techniques but failed. It was the longest war waged by East India Company during their military campaigns on the Indian subcontinent. It was one of the bloodiest and hardest wars waged by East India Company in India with Presidency army regiments that suffered losses as high as eighty percent in 10 years of warfare.

The Dominican Restoration War was a guerrilla war between 1863 and 1865 in the Dominican Republic between nationalists and Spain, the latter of which had recolonized the country 17 years after its independence. The war resulted in the withdrawal of Spanish forces and the establishment of a second republic in the Dominican Republic.

The Moroccan military leader Abd el-Krim (c. 1883 – 1963) and his father unified the Moroccan tribes under their control and took up arms against the Spanish and French occupiers during the Rif War in 1920. For the first time in history, tunnel warfare was used alongside modern guerrilla tactics, which caused considerable damage to both the colonial armies in Morocco.
In the early 20th century Michael Collins and Tom Barry both developed many tactical features of guerrilla warfare during the guerrilla phase of the 1919–1921 Irish War of Independence. Collins developed mainly urban guerrilla warfare tactics in  Dublin City (the Irish capital). Operations in which small Irish Republican Army (IRA) units (3 to 6 guerrillas) quickly attacked a target and then disappeared into civilian crowds frustrated the British enemy. The best example of this occurred on Bloody Sunday (21 November 1920), when Collins's assassination unit, known as "The Squad", wiped out a group of British intelligence agents ("the Cairo Gang") early in the morning (14 were killed, six were wounded) – some regular officers were also killed in the purge. That afternoon, the Royal Irish Constabulary force consisting of both regular RIC personnel and the Auxiliary Division took revenge, shooting into a crowd at a football match in Croke Park, killing fourteen civilians and injuring 60 others.
In West County Cork, Tom Barry was the commander of the IRA West Cork brigade. Fighting in west Cork was rural, and the IRA fought in much larger units than their fellows in urban areas. These units, called "flying columns", engaged British forces in large battles, usually for between 10 – 30 minutes. The Kilmichael Ambush in November 1920 and the Crossbarry Ambush in March 1921 are the most famous examples of Barry's flying columns causing large casualties to enemy forces.

The Algerian Revolution of 1954 started with a handful of Algerian guerrillas. Primitively armed, the guerrillas fought the French for over eight years. This remains a prototype for modern insurgency and counterinsurgency, terrorism, torture, and asymmetric warfare prevalent throughout the world today. In South Africa, African National Congress (ANC) members studied the Algerian War, prior to the release and apotheosis of Nelson Mandela; in their intifada against Israel, Palestinian fighters have sought to emulate it. Additionally, the tactics of Al-Qaeda closely resemble those of the Algerians.
The Mukti Bahini (Bengali: মুক্তিবাহিনী, translates as "freedom fighters", or liberation army), also known as the Bangladesh Forces, was the guerrilla resistance movement consisting of the Bangladeshi military, paramilitary and civilians during the Bangladesh Liberation War that transformed East Pakistan into Bangladesh in 1971. An earlier name Mukti Fauj was also used.

Theoretical works: The growth of guerrilla warfare 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:...our troops should...fight while protected by the terrain...using small, mobile guerrilla units to exhaust the enemy...denying them rest so that they only control the terrain under their feet.
More recently, Mao Zedong's On Guerrilla Warfare, Che Guevara's Guerrilla Warfare, and Lenin's Guerrilla warfare, were all written after the successful revolutions carried out 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".

Foco theory: 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 "focal-ism" 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.

Strategy, tactics and methods: 

Strategy: Guerrilla warfare is a type of asymmetric warfare: competition between opponents of unequal strength.