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:
- 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 do not satisfy these criteria are termed "irregular forces".