Common Law was changed a very long time ago. The point is, if you do not like the law you change it, not subvert it.
Back to the Geneva Convention. Soldiers are encouraged to follow these rules against their enemies in the hope that if they are captured, they will be accorded the same consideration. We do our servicemen an injustice when we don't follow these rules.
Nonuniformed combatants are treated as spies, saboteurs, etc. and MAY be executed on the spot by a military tribunal, irregardless of citizenship and location.