Someone who fights, especially in a war, is a combatant. Sometimes entire countries are described as combatants during military conflicts.
Other names for a combatant are "fighter," "attacker," or "soldier." News stories are more likely to use this word to describe an adversary or enemy: "Enemy combatants ignored the cease fire." When they're talking about their own country's fighters, they might say "troops" or "soldiers." Combatant comes from the Old French combatant, "skilled at fighting" or "warlike."