I wish to pass no judgment on what actually happened at Guadalcanal, but in the movie the Americans were taunting in their victory, exacting revenge on the Japanese. It was in this way that the movie showed neither side was in true possession of the pure goodness of humanity. War isn't just thrusting a sword at someone or slowly aiming the turret of a tank - it's about the decisions people made in order to get to that war and what they will do in that war. War is the ultimate morality play, because the stakes are so high. All of the things that go on in a war, from the killing to the plotting against the enemy to espionage, all of it could be translated in some way to something that happens in everyday reality.
People kill in peacetime, people plot against others for personal gain in peacetime.
The simple act of watching and gossiping can be called a red-headed stepchild to espionage. So war is just a sort of violent extension of what so many normal people do everyday. There was an Edenic setting in The Thin Red Line, where life was basically simple and happy. Granted, the people there lived fairly primitively, but they got along in life well, without too much hardship. Somehow, these people had forged themselves an island of respite from the failures of the outsiders.