Irregular warfare is defined in United States joint doctrine as "a violent struggle among state and non-state actors for legitimacy and influence over the relevant populations." Concepts associated with irregular warfare are older than the term itself.
One of the earliest known uses of the term irregular warfare is in the 1986 English edition of "Modern Irregular Warfare in Defense Policy and as a Military Phenomenon" by former Nazi officer Friedrich August Freiherr von der Heydte. The original 1972 German edition of the book is titled "Der Moderne Kleinkrieg als Wehrpolitisches und Militarisches Phänomen". The German word "Kleinkrieg" is literally translated as "Small War". The word "Irregular", used in the title of the English translation of the book, seems to be a reference to non "regular armed forces" as per the Third Geneva Convention.
One of the earliest known uses of the term IW is in a 1996 Central Intelligence Agency document by Jeffrey B. White. Major military doctrine developments related to IW were done between 2004 and 2007 as a result of the September 11 attacks on the United States. A key proponent of IW within US DoD is Michael G. Vickers, a former paramilitary officer in the CIA. The CIA's Special Activities Division (SAD) is the premiere unit for unconventional warfare, both for creating and for combating irregular warfare units. For example, SAD paramilitary officers created and led successful irregular units from the Hmong tribe during the war in Laos in the 1960s from the Northern Alliance against the Taliban during the war in Afghanistan in 2001 and from the Kurdish Peshmerga against Ansar al-Islam and the forces of Saddam Hussein during the war in Iraq in 2003.
Another former CIA Paramilitary Operations Officer and Deputy Secretary of Defense Mick Mulroy said at an October 2019 workshop at RAND that the Irregular Warfare Annex is a critical component of the 2018 National Defense Strategy. He said irregular warfare (IW) included counter-insurgency, counter-terrorism, unconventional warfare, foreign internal defense, sabotage and subversion, as well as stabilization and information operations. It had traditionally been perceived as a predominately counterterrorism (CT) effort used to fight violent extremist organizations but it should be applied to all areas of military competition and against global powers competitors like China and Russia as well as rogue states like North Korea and Iran. Mulroy said that the U.S. must be prepared to respond with "aggressive, dynamic, and unorthodox approaches to IW," to be competitive.
Irregular warfare favors indirect warfare and asymmetric warfare approaches, though it may employ the full range of military and other capabilities, in order to erode the adversary's power, influence, and will. It is inherently a protracted struggle that will test the resolve of a state and its strategic partners. The distinction between regular and irregular forces is unrelated to the term "irregular warfare." The term, irregular warfare, was settled upon in distinction from "traditional warfare" and "unconventional warfare", and to differentiate it as such.
- IW is a form of warfare that has as its objective the credibility and/or legitimacy of the relevant political authority with the goal of undermining or supporting that authority. IW favors indirect approaches, though it may employ the full range of military and other capabilities to seek asymmetric approaches, in order to erode an adversary's power, influence, and will.
- IW is defined as a violent struggle among state and non-state actors for legitimacy and influence over the relevant population(s)
- IW involves conflicts in which enemy combatants are not regular military forces of nation-states.
- IW is "war among the people" as opposed to "industrial war" (i.e. regular war).