Refers to an individual person who takes part in combat, especially in a military context.
Under international law, it is necessary to distinguish between combatants and non-combatants.
belligerent; warring nation; party to a conflict
Refers to a state, nation, or organized group that is engaged in warfare, often used in legal or diplomatic contexts.
The belligerents began peace negotiations.
交戦国 specifically means 'belligerent nation' and is used for states at war. 交戦者 can refer to any party to a conflict, including non-state actors.
Compound of 交戦 (kōsen, 'warfare, hostilities') + 者 (sha, 'person, party'). The term is a straightforward Sino-Japanese formation.