in terms of us and them" is Pat Churchland's basic hypothesis - multi-disciplinary (see Churchland links in my post above). Her evolutionary path boils down to:

Me

Me-Mine (kids)

Me-Mine-Kin

Me-Mine-Kin-Kith
As to "trusting" strangers, the quickest method is a trusted intermediary known to both parties. All of us probably know the non-intermediary method: put the trade goods on the beach; and wait for the other guy's trade goods offered in exchange. Recycle until someone picks up the last offered goods. That is a more time-consuming process.

Here is some more of Jon Haidt on a similar topic:

Johnathan Haidt, The Groupish Gene (2012) (short course, 17 min.)
http://www.youtube.com/watch?v=T64_El2s7FU

Johnathan Haidt, The Groupish Gene (2012) (long course, 1.5 hrs)
http://www.youtube.com/watch?v=NQ192d4c4S0

A lot of scientific weeds grow here, just as a lot of legal weeds grow elsewhere. The bottom line is that "we-they" is a human group given, but hypotheses differ on whether that given is caused by cultural selection, genetic selection, or both (in which case, how much from each); and, in both types of selection, the relative roles played by individuals and groups.

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I don't want to get down into the legal weeds - and simply won't; but this is a muddled mess:

As to the distinction between Law of War and Rule of Law, I would assert that in the domain of "Legal Processes" the former is a subset of the latter. In other words, it really makes no sense to talk about the Law of War unless an environment exists in which some fair/objective process (or perhaps "due process" is a better word choice) exists to examine whether aspects of the Law of War have been followed or breached.

This last may seem to be at variance with your points about an absence of the rule of law, but I took your point to be that this absence is found in the area of operations, not in the organization conducting COIN. The deployed force has a goal (perhaps) of installing or restoring the rule of law to that place where the COIN mission is being conducted; that deployed force also operates internally under the Rule of Law. In the case at hand, the UCMJ/MCM is a significant part of the Rule of Law specification but is not the whole story.
Are you arguing something along the lines of Kevin Heller's hypothesis, 'One Hell of a Killing Machine': Signature Strikes and International Law (to be published) ?

Perhaps you could diagram your "Legal Domain", and how all these fit together.

Regards

Mike