The term belligerent was used until the end of World War II to refer to:
This term no longer has a precise legal definition. Instead, the term party to the conflict is used since 1977 to define both State and non-state actors participating in an armed conflict.
However, the term belligerent continued to be used until 1977 to refer to individual insurgents who, in a civil war, actually controlled part of the territory of a State. So as to ensure a more cohesive system of protection and to simplify the distinction between the civilian population and those fighting in an armed conflict, the term combatant is now used in humanitarian law (API Arts. 43, 44, 48). The word belligerent is still used in everyday language.
For Additional Information: Dinstein, Yoram. The Conduct of Hostilities under the Law of International Armed Conflict . Cambridge: Cambridge University Press, 2004.
Henckaerts, Jean-Marie, and Louise Doswald-Beck, eds. Customary International Law . Vol. 1, The Rules . Cambridge: Cambridge University Press, 2005, esp. parts 2 and 5.