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Irregular military is any non-standard military, that is, distinct from that of the regular army. 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 troops, irregular infantry and irregular cavalry.
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
"Regular" and "irregular" have been used to describe combat forces for hundreds of years, usually with little ambiguity. Due to a government's chain of command requirements, the regular army is very well defined, and anybody fighting outside of that (excluding official paramilitary forces) are irregulars. In cases where the legitimacy of the army or its opponents is questioned, some legal definitions have been created.
In international humanitarian law, "irregular forces" refers to a category of combatants consisting of individuals forming part of the armed forces of a party to an armed conflict, international or non-international, but not belonging to that party's regular forces and operating in 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:
- being commanded by a person responsible for his subordinates to a party of conflict
- having a fixed distinctive emblem recognizable at a distance
- carrying arms openly
- conducting operations in accordance with the laws and customs of war
By extension, combat forces that don't satisfy these criteria are "irregular forces".
Other names for irregular military formations
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The term "irregular military" describes the "how" and "what", but it is more common to focus on the "why". 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 in and out of fashion, based on political and emotional associations that develop. Here is a list of such terms, organized more or less from oldest to latest.
- Auxiliaries - foreign or allied troops supplementing the regular army, organized from provincial or tribal regions. In the Imperial Roman army it became common to maintain a number of auxiliaries about equal to the legionaries.
- Levies - feudal peasants and freemen liable to be called up for short-term military duty.
- Privateers - a "for-profit" private person or ship authorized and sponsored by a government by letters of marque to both attack foreign vessels during wartime and destroy or disrupt logistics of the enemy during "peacetime", often on the open sea by attacking its merchant shipping, rather than engaging its combatants or enforcing a blockade against them.
- Revolutionary — someone part of a revolution, whether military or not.
- Guerrilla — someone who uses unconventional military tactics, tends to refer to groups engaged in open conflict rather than underground resistance. Term coined during the Peninsula War in Spain against France.
- Franc-tireur — French irregular forces during the Franco-Prussian War. But is also used in international legal cases as a synonym for unprivileged combatant (see for example the Hostages Trial [1947—1948]).
- Militia — military force composed of ordinary citizens.
- Partisan — In the 20th century, someone part of a resistance movement. In the 18th and 19th century, a local conventional military force using irregular tactics. Often used to refer to resistance movements against the Axis Powers during the Second World War.
- Freedom fighter — type of irregular military which the main cause, in their or their supporters' view, is freedom for themselves or others.
- Paramilitary — non-regular Armed Force with a claim to official status.
- Terrorist — irregular military who target civilians and other non-combatants in order to gain political leverage; this term is almost always used pejoratively, and is, like the term freedom fighter, very subjective.
- False flag or pseudo-operations where troops of one side dress like another to eliminate or discredit them and their support, such as members of the Panzer Brigade 150 commanded by Waffen-SS commando Otto Skorzeny in Operation Greif during the Battle of the Bulge in World War II and Selous Scouts of the Rhodesian Bush War.
- Insurgent — an alternate term for a member of an irregular military. Tends to refer to members of underground groups such as the Iraqi Insurgency rather than larger rebel organizations such as the Revolutionary Armed Forces of Colombia.
Intense debates can build up over which of these terms to use when referring to a specific group. Using one term over another can imply either strong support or opposition for the cause being fought over.
It is possible for a military to cross the line between regular and irregular. Isolated regular army units forced to operate without regular support for long periods of time can degrade into irregulars. As an irregular military becomes more successful, they may transition away from irregular, even to the point of becoming the new regular army if they win.
Regular military units which use irregular military tactics
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Most conventional military officers and militaries are wary of using irregular military forces seeing them as unreliable, of doubtful military usefulness, and prone to committing atrocities which lead to retaliation in kind. Usually such forces are raised outside the regular military for example the British SOE during World War II, and the CIA's Special Activities Division more recently. However at times, sometimes out of desperation, conventional militaries will resort to guerilla tactics, usually to buy breathing space and time for themselves through tying up enemy forces by threatening their line of communications and rear areas, for example the 43rd Battalion Virginia Cavalry and the Chindits.
Although they are part of a regular army, United States Special Forces are trained in, amongst other missions, to implement irregular military tactics. However, outside the United States the term special forces does not generally imply a force trained to fight as guerillas and insurgents. Originally, 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 and elsewhere. United States Special Forces and the CIA's Special Activities Division can trace their lineage to the OSS operators in World War II, who were tasked with inspiring, training, arming and leading resistance movements in Nazi occupied Europe and Japanese occupied Asia.
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
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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 legions 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 legions would equal the regular legions; however, since irregulars won that battle, there are few surviving details.
During the decline of the Roman Empire, irregulars made up an ever increasing proportion of the Roman military. At the end, there was little difference between the Roman military and the barbarians across the borders. Throughout history, most civilizations eventually fell to "barbarians", that is, irregular military forces, with minimal historical details.
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. Prior to 1857 Britain's Honourable East India Company maintained large numbers of cavalry and infantry regiments officially designated as "irregulars". These were less formally drilled and had fewer British officers than the "regular" sepoys in British service.
Use of large irregular forces featured heavily in wars such as the American Revolution, the Irish War of Independence, the Franco-Prussian War, the Russian Civil War, the Second Boer War, Vietnam War, and especially the Eastern Front of World War II where hundreds of thousands of partisans fought on both sides.
The 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.
Examples of irregular military
- Bands (Italian Army irregulars)
- Bashi-bazouk — Irregular mounted mercenary in the Ottoman Empire
- Camisards — Huguenot insurgency in the beginning of the 17th century in the Cévennes
- Filibuster (military)
- Hajduks— bandits and irregulars in and against the Ottoman Empire, but found amongst military ranks in Hungary and the Polish–Lithuanian Commonwealth
- Harkis; Algerian Muslim irregulars who served with the French Army during the Algerian War of 1954-62.
- Klephts — Greek guerrilla fighters in Ottoman Greece
- Legion of Frontiersmen — An irregular quasi-military organization that proliferated throughout the British Empire prior to World War I
- Makhnovshchina — Ukrainian anarchist army that fought both the White Armies and the Bolsheviks during the Russian Civil War.
- Minutemen — American irregular troops during the American Revolution
- Trenck's Pandurs — Habsburg Monarchy 17th and 18th century skirmisher, later evolving in the regular Grenz infantry.
- Partisan (military)
- Pindari — 18th century irregular horsemen in India
- Landsturm — created by a 21 April 1813 edict of Frederick William III of Prussia
- Righteous Army— in Korean history
- Rough Riders — in the Spanish–American War
- Viet Cong
Irregulars in modern 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 war in Vietnam in the 1960s. 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.
- Fifth column
- Resistance movement
- "Yank" Levy teacher of the Home Guard and coauthor of the first practical book on Guerrilla Warfare
- Modern concepts
- Asymmetric warfare — Military theory that also includes regulars vs. irregulars
- Fourth generation warfare
- Guerrilla warfare
- Irregular warfare
- Low intensity conflict
- Unconventional warfare
- Violent non-state actors
- Flavius Vegetius Renatus, Epitoma rei militaris
- Dr. Thomas M. Huber, Compound Warfare: An Anthology
- Clifford J. Rogers, Military Technical Revolution debate among historians
- John M. Gates, US Army & Irregular Warfare
- Harold P. Ford, CIA and the Vietnam Policymakers: Three Episodes 1962-1968
- Robert R. Mackey, "The UnCivil War: Irregular Warfare in the Upper South, 1861-1865," University of Oklahoma Press, 2004, ISBN 0-8061-3624-3
- Boczek, Boleslaw Adam, "International law: a dictionary", ISBN 0-8108-5078-8, ISBN 978-0-8108-5078-1, Scarecrow Press, 2005 
- Bybee, Jay S., "Status of Taliban Forces Under Article 4 of the Third Geneva Convention of 1949", 7 February 2002 
- U.S. Special Forces: A Guide to America's Special Operations Units: the World's Most Elite Fighting Force; by Samuel A. Southworth, Stephen Tanner, Published by Da Capo Press, 2002, ISBN 0-306-81165-0
- The CIA Secret Army, publisher Time Inc, Douglas Waller, 2003-02-03
- All Necessary Means: Employing CIA Operatives in a Warfighting Role Alongside Special Operations Forces, Colonel Kathryn Stone, Professor Anthony R. Williams (Project Advisor), United States Army War College (USAWC), 7 April 2003.
- Shooting at the Moon: The Story of America's Clandestine War in Laos, Steerforth Press, 1996 ISBN 978-1-883642-36-5
- Crile, George (2003). Charlie Wilson's War: The Extraordinary Story of the Largest Covert Operation in History. Atlantic Monthly Press. ISBN 0-87113-854-9.
- Woodward, Bob Bush at War, Simon and Schuster, 2002
- Operation Hotel California: The Clandestine War inside Iraq, Mike Tucker, Charles Faddis, 2008, The Lyons Press ISBN 978-1-59921-366-8
- Woodward, Bob Plan of Attack, Simon and Schuster, 2004 ISBN 978-0-7432-5547-9