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A belligerent (lat. bellum gerere, "to wage war") is an individual, group, country, or other entity that acts in a hostile manner, such as engaging in combat. Belligerent comes from Latin, literally meaning "one who wages war".  Unlike the use of belligerent as an adjective to mean aggressive, its use as a noun does not necessarily imply that a belligerent country is an aggressor.
In times of war, belligerent countries can be contrasted with neutral countries, and non-belligerents. However, the application of the laws of war to neutral countries and the responsibilities of belligerents are not affected by any distinction between neutral countries, neutral powers or non-belligerents — a non-belligerent may nevertheless risk being considered a belligerent if it aids or supports a belligerent in a way that is proscribed by neutral countries.
An interesting use of the term arose during the American Civil War, when the Confederate States of America, though not recognized as a sovereign state, was recognized as a belligerent power; and, as a consequence, Confederate warships were given the same rights as US warships in foreign ports.   
"Belligerency" is a term used in international law to indicate the status of two or more entities, generally sovereign states, being engaged in a war. Wars are often fought with one or both parties to a conflict invoking the right to self-defence under Article 51 of the United Nations Charter  (as the United Kingdom did in 1982 before the start of the Falklands War ) or under the auspices of a United Nations Security Council resolution (such as the United Nations Security Council Resolution 678, which gave legal authority for the Gulf War).
A state of belligerency may also exist between one or more sovereign states on one side and rebel forces, if such rebel forces are recognised as belligerents. If there is a rebellion against a constituted authority (for example, an authority recognised as such by the United Nations), and those taking part in the rebellion are not recognized as belligerents, the rebellion is an insurgency.  Once the status of belligerency is established between two or more states, their relations are determined and governed by the laws of war. 
- Present participle belligerent- (nominative singular belligerēns).
- Hall, Kermit L. The Oxford Guide to United States Supreme Court Decisions, Oxford University Press US, 2001 ISBN 0-19-513924-0, ISBN 978-0-19-513924-2 p. 246 "In supporting Lincoln on this issue, the Supreme Court upheld his theory of the Civil War as an insurrection against the United States government that could be suppressed according to the rules of war. In this way the United States was able to fight the war as if it were an international war, without actually having to recognize the de jure existence of the Confederate government."
- Staff. Bureau of Public Affairs: Office of the Historian -> Timeline of U.S. Diplomatic History -> 1861-1865:The Blockade of Confederate Ports, 1861-1865, U.S. State Department. "Following the U.S. announcement of its intention to establish an official blockade of Confederate ports, foreign governments began to recognize the Confederacy as a belligerent in the Civil War. Great Britain granted belligerent status on May 13, 1861, Spain on June 17, and Brazil on August 1. Other foreign governments issued statements of neutrality."
- Goldstein, Erik; McKercher, B. J. C. Power and stability: British foreign policy, 1865-1965, Routledge, 2003 ISBN 0-7146-8442-2, ISBN 978-0-7146-8442-0. p. 63
- Chapter VII — Action with respect to Threats to the Peace, Breaches of the Peace, and Acts of Aggression United Nations website, 23 August 2016
- Daniel K. Gibran (1997). The Falklands War: Britain Versus the Past in the South Atlantic, McFarland, ISBN 0-7864-0406-X. p. 86
- Oxford English Dictionary second edition 1989 "insurgent B. n. One who rises in revolt against constituted authority; a rebel who is not recognized as a belligerent."
- Chisholm, Hugh, ed. (1911). Encyclopædia Britannica. 3 (11th ed.). Cambridge University Press. .