Why are there still officers and enlisted in modern militaries, as opposed to just one rank hierarchy? I'm guessing the origin of the system goes back to knights and feudal times (maybe?), and anecdotally I've heard that before everyone was literate, officers read orders aloud to their men. I'm in a U.S. officer accession program, and from what I've experienced, the distinction in practice is just one of class -- who can afford to go to college, etc.
The Geneva Convention requires that all combatants must be commanded by an officer. The main reason is the officer can be held legally responsible for the actions of those he commands.
If you look at the Nuremberg Trials; all the men tried for war crimes were senior officers, high ranking members of government, or private citizens. So even though enlisted soldiers carried out many of war crimes, the ultimate blame rests with the commanders.
That not to say enlisted men are immune from war crime charges, however when you look at the My Lai Massacre during the Vietnam War, many enlisted participated in killings and were charged but only their platoon leader, Lieutenant William Calley Jr, was convicted.