Warrior is still a warrior when fighting alone, right away from a unit. Warriors can fight in units, but they can also fight alone. Soldier has a much more complicated etymology where we get that word a it literally means one serving for pay. When the war ends, a soldier, because it's a job, may go back to being a civilian. We have now a broken the taboo against having former or active military officers as secretary of defense twice in four years. A there seems to be a real sense that only soldiers should sit as secretary defence - which used to be only civilians should.