"Rules of engagement" are less noble than they sound. They are not internationally reviewed and approved, they are unilaterally imposed by one party.
An infamous example being the "do not approach within 50 metres of this vehicle" for convoy escorts in Iraq. Under no international law can the use of lethal force be condoned in that circumstance yet it was 'legal' under the RoE.
And I'm not saying that as some lefty pacifist, having been in uniform myself.