Polarbear: As I read through the above posts most everyone speaks or at least implies that establishing the rule law in a counter insurgence is a complicated business. My gut than screams, well then, SIMPLIFY IT.
The law of war (or law of armed conflict as it is sometimes called) doesn't tell you how to address a land dispute issue between two farmers. It doesn't tell jurisdiction or composition of a court system. It doesn't demonstrate the difference between tax collection and a shakedown. Simply put, the Law of War tells you nothing about Rule of Law.

Law of war dictates how war is fought. Rule of law is an ideal by which a country should function (at least from a western perspective). I think you confuse the two. There is a law of occupation that governs how an occupying country (presumably with a military force) must administer an occupied country, but it doesn't apply in Afghanistan because there is a sovereign, functioning government here.

The question becomes how well do we wish the Afghan government to function? Right now, it works, but not very well. Corruption is systemic, governmental control is tenuous, at best, in some parts of the country, and the educational level of most of the country precludes immediate solutions. This is why the effort is complex. We could simplify, say, criminal law within Afghanistan, but it would not be compliant with typical notions of rule of law or Afghan commitments under international law. Hell, the Taliban had a functioning criminal justice system that was efficient, fast, and provided some form of justice. It did not, however, equate to rule of law; rather, it was rule by law.

Why is rule of law important? For a society to function effectively and provide for the well-being of its people, rule of law is crucial. Rule of law ensures that a country is a nation of law rather than of men. In other words, it requires that all are subject to a non-arbitrary legal system. This, in turn, allows people to plan for the future. They can start a business, buy property, etc. Rule of law encourages investment, something Afghanistan sorely needs, because property rights and contract law (components of rule of law) are crucial to the attraction of foreign investment.

The law of war looks to the present; it says how a war is supposed to be fought. Combatants and civilians are given rights that are only applicable during the war so that war's ferocity can be limited in some fashion. Rule of law, however, looks to the future. It seeks to help a country, particularly a warn-torn country, get back on its feet and work toward a viable future. The absence of rule of law merely perpetuates the war paradigm; it allows men to rule by law (force) and make a country their own little fiefdom. Rule of law cuts against this by establishing rules of conduct that respect each individual's rights and freedoms. Again, it provides a degree of certainty.

From your first post:
insurgents are a very small step above spies
it isn't factually or legally correct. While there are indeed differences of opinion among legal folks, an insurgent is invariably seen as a combatant (legal or otherwise). A spy is in a quite different category. The Geneva Conventions recognize only two categories of persons on the battlefield: combatants and non-combatants. A combatant is one who takes up arms; a non-combatant does not do so. A combatant that violates the laws of war, as do insurgents that use the tactic of terrorism, remains a combatant. Sure, Bush lawyers invented the term "illegal combatant," but this isn't found in the Geneva Conventions. A combatant that violates the laws of war loses his criminal immunity and can be tried for war crimes. The legal limbo in which our guests at Gitmo are in is a new invention, but that's another story.

ROE that are based on the Laws of War
While ROE must comply with the laws of war, they are not "based" on it. Laws of war are just that, laws. They have become customary international law and, thus, must be obeyed by those engaging in warfighting. Rules of engagement are just that, rules. These rules are established with command authority to govern action on the battlefield in such a way as to give some measure of control to command authorities that may not actually be present on the battlefield. This is the classic political dominance over military means (Clausewitz points this out by recognizing that war is a continuation of politics). These rules permit politicos to maintain control over the means they use to obtain a political ends by ensuring that those means do not create additional political obstacles. Sometimes this is successful, sometimes not (e.g. Abu Ghraib).

How do we handle insurgents who commit war crimes? To turn them over to civilian host nation authorities ignores the war crimes and does little to stop civilian deaths.
This is being done here, to some extent. There is a national security court system here that prosecutes those captured on the battlefield. These Afghans are tried in an Afghan court, on Afghan soil, by an Afghan prosecutor, while represented by an Afghan defense attorney, in a trial presided by a panel of Afghan judges. American attorneys do play an advisory role, but have no active role in the proceedings.