Irregular military

Irregular military is any non-standard military component that is distinct from a country's national armed forces. Being defined by exclusion, there is significant variance in what comes under the term. It can refer to the type of military organization, or to the type of tactics used. An irregular military organization is one which is not part of the regular army organization. Without standard military unit organization, various more general names are often used; such organizations may be called a "troop", "group", "unit", "column", "band", or "force". Irregulars are soldiers or warriors that are members of these organizations, or are members of special military units that employ irregular military tactics. This also applies to irregular infantry and irregular cavalry units.
Irregular warfare is warfare employing the tactics commonly used by irregular military organizations. This involves avoiding large-scale combat, and focusing on small, stealthy, hit-and-run engagements.
Regular vs. irregular.
The words "regular" and "irregular" have been used to describe combat forces for hundreds of years, usually with little ambiguity. The requirements of a government's chain of command cause the regular army to be very well defined, and anybody fighting outside it, other than official paramilitary forces, are irregular. In case the legitimacy of the army or its opponents is questioned, some legal definitions have been created.
In international humanitarian law, the term "irregular forces" refers to a category of combatants that consists of individuals forming part of the armed forces of a party to an armed conflict, international or domestic, but not belonging to that party's regular forces and operating inside or outside of their own territory, even if the territory is under occupation.
The Third Geneva Convention of 1949 uses "regular armed forces" as a critical distinction. The International Committee of the Red Cross (ICRC) is a non-governmental organization primarily responsible for and most closely associated with the drafting and successful completion of the Third Geneva Convention Relative to the Treatment of Prisoners of War ("GPW"). The ICRC provided commentary saying that "regular armed forces" satisfy four Hague Conventions (1899 and 1907) (Hague IV) conditions. In other words, "regular forces" must satisfy the following criteria:
By extension, combat forces that do not satisfy these criteria are termed "irregular forces".
Other names for irregular military formations.
The term "irregular military" describes the "how" and "what", but it is more common to focus on the "why" as just about all irregular units were created to provide a tactical advantage to an existing military, whether it was privateer forces harassing shipping lanes against assorted New World colonies on behalf of their European contractors, or Auxiliaries, levies, civilian and other standing irregular troops that are used as more expendable supplements to assist costly trained soldiers. Bypassing the legitimate military and taking up arms is an extreme measure. The motivation for doing so is often used as the basis of the primary label for any irregular military. Different terms come into and out of fashion, based on political and emotional associations that develop. Here is a list of such terms, which is organized more or less from oldest to latest:
Intense debates can build up over which term is to be used to refer to a specific group. Using one term over another can strongly imply strong support or opposition for the cause.
It is possible for a military to cross the line between regular and irregular. Isolated regular army units that are forced to operate without regular support for long periods of time can degrade into irregulars. As an irregular military becomes more successful, it may transition away from irregular, even to the point of becoming the new regular army if it wins.
Regular military units that use irregular military tactics.
Most conventional military officers and militaries are wary of using irregular military forces and see them as unreliable, of doubtful military usefulness and prone to committing atrocities leading to retaliation in kind. Usually, such forces are raised outside the regular military like the British SOE during World War II and, more recently, the CIA's Special Activities Division. However at times, such as out of desperation, conventional militaries will resort to guerilla tactics, usually to buy breathing space and time for themselves by tying up enemy forces to threaten their line of communications and rear areas, such as the 43rd Battalion Virginia Cavalry and the Chindits.
Although they are part of a regular army, United States Special Forces are trained in missions such as implementing irregular military tactics. However, outside the United States, the term special forces does not generally imply a force that is trained to fight as guerillas and insurgents. Originally, the United States Special Forces were created to serve as a cadre around which stay-behind resistance forces could be built in the event of a communist victory in Europe or elsewhere. The United States Special Forces and the CIA's Special Activities Division can trace their lineage to the OSS operators of World War II, which were tasked with inspiring, training, arming and leading resistance movements in German-occupied Europe and Japanese occupied Asia.
In Finland, well-trained light infantry sissi troops use irregular tactics such as reconnaissance, sabotage and guerrilla warfare behind enemy lines.
Effectiveness.
While the morale, training and equipment of the individual irregular soldier can vary from very poor to excellent, irregulars are usually lacking the higher-level organizational training and equipment that is part of regular army. This usually makes irregulars ineffective in direct, main-line combat, the typical focus of more standard armed forces. Other things being equal, major battles between regulars and irregulars heavily favor the regulars.
However, irregulars can excel at many other combat duties besides main-line combat, such as scouting, skirmishing, harassing, pursuing, rear-guard actions, cutting supply, sabotage, raids, ambushes and underground resistance. Experienced irregulars often surpass the regular army in these functions. By avoiding formal battles, irregulars have sometimes harassed high quality armies to destruction. 
The total effect of irregulars is often underestimated. Since the military actions of irregulars are often small and unofficial, they are underreported or even overlooked. Even when engaged by regular armies, some military histories exclude all irregulars when counting friendly troops, but include irregulars in the count of enemy troops, making the odds seem much worse than they were. This may be accidental; counts of friendly troops often came from official regular army rolls that exclude unofficial forces, while enemy strength often came from visual estimates, where the distinction between regular and irregular were lost. If irregular forces overwhelm regulars, records of the defeat are often lost in the resulting chaos. 
Historical reliance on irregulars.
By definition, "irregular" is understood in contrast to "regular armies," which grew slowly from personal bodyguards or elite militia. In Ancient warfare, most civilized nations relied heavily on irregulars to augment their small regular army. Even in advanced civilizations, the irregulars commonly outnumbered the regular army.
Sometimes entire tribal armies of irregulars were brought in from internal native or neighboring cultures, especially ones that still had an active hunting tradition to provide the basic training of irregulars. The regulars would only provide the core military in the major battles; irregulars would provide all other combat duties.
Notable examples of regulars relying on irregulars include Bashi-bazouk units in the Ottoman Empire, auxiliary cohorts of Germanic peoples in the Roman Empire, Cossacks in the Russian Empire, and Native American forces in the American frontier of the Confederate States of America.
One could attribute the disastrous defeat of the Romans at the Battle of the Teutoburg Forest to the lack of supporting irregular forces; only a few squadrons of irregular light cavalry accompanied the invasion of Germany when normally the number of "foederati" and auxiliaries would equal the regular legions. During this campaign the majority of locally recruited irregulars defected to the Germanic tribesmen led by the former auxiliary officer Arminius.
During the decline of the Roman Empire, irregulars made up an ever-increasing proportion of the Roman military. At the end of the Western Empire, there was little difference between the Roman military and the barbarians across the borders. 
Following Napoleon's modernisation of warfare with the invention of conscription, the Peninsular War led by Spaniards against the French invaders in 1808 provided the first modern example of guerrilla warfare. Indeed, the term of "guerrilla" itself was coined during this time.
As the Industrial Revolution dried up the traditional source of irregulars, nations were forced take over the duties of the irregulars using specially trained regular army units. Examples are the light infantry in the British Army.
Irregular regiments in British India.
Prior to 1857 Britain's East India Company maintained large numbers of cavalry and infantry regiments officially designated as "irregulars", although they were permanently established units. These were less formally drilled and had fewer British officers (sometimes only three or four per regiment) than the "regular" sepoys in British service. This system enabled the Indian officers to achieve greater responsibility than their counterparts in regular regiments. Promotion for both Indian and British officers was for efficiency and energy, rather than by seniority as elsewhere in the EIC's armies. In irregular cavalry the Indian troopers provided their horses under the "silladar" system. The result was a loose collection of regiments which in general were more effective in the field, if not so smart on parade, than their regular counterparts. These irregular units were also cheaper to raise and maintain and as a result many survived into the new Indian Army that was organized following the great Indian Rebellion of 1857.
Irregular military in Canada before 1867.
Before 1867, military units in Canada consisted of British units of volunteers.
During French rule, small local volunteer militia units or colonial militias were used to provide defence needs. During British control of various local militias, the Provincial Marine were used to support British regular forces in Canada.
Other instances of irregulars.
Use of large irregular forces featured heavily in wars such as the American Revolution, the Irish War of Independence and Irish Civil War, the Franco-Prussian War, the Russian Civil War, the Second Boer War, Liberation war of Bangladesh, Vietnam War, the Syrian Civil War and especially the Eastern Front of World War II where hundreds of thousands of partisans fought on both sides.
The Chinese People's Liberation Army began as a peasant guerilla force which in time transformed itself into a large regular force. This transformation was foreseen in the doctrine of "people's war", in which irregular forces were seen as being able to engage the enemy and to win the support of the populace but as being incapable of taking and holding ground against regular military forces.
Irregulars in today's warfare.
The ongoing conflicts of post-invasion Iraq, the renewed Taliban insurgency in the 2001 war in Afghanistan, the Darfur conflict, the rebellion in the North of Uganda by the Lord's Resistance Army, and the Second Chechen War are fought almost entirely by irregular forces on one or both sides.
The CIA's Special Activities Division (SAD) is the premiere United States unit for creating or combating irregular military forces. SAD paramilitary officers created and led successful units from the Hmong tribe during the Laotian Civil War in the 1960s and 1970s. They also organized and led the Mujaheddin as an irregular force against the Soviet Union in Afghanistan in the 1980s, as well as the Northern Alliance as an irregular insurgency force against the Taliban with US Army Special Forces during the war in Afghanistan in 2001 and organized and led the Kurdish Peshmerga with US Army Special Forces as an irregular counter-insurgency force against the Kurdish Sunni Islamist group Ansar al-Islam at the Iraq-Iran border and as an irregular force against Saddam Hussein during the war in Iraq in 2003.
References.
General references:
Specific references: