Irregular warfare (IW) 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.
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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.
Another early use of the term 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[clarification needed], 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.
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.
In an entry to the electronic open access Handbook of Military Sciences the Dutch military scholar Martijn Kitzen explores Operations in Irregular Warfare and the underlying body of knowledge which characterizes these conflicts as violent struggles involving non-state actors and states that seek to establish power, control, and legitimacy over relevant populations. In the chapter, Kitzen provides an overview of much of the academic literature that covers this field.