I disagree with the (2). The laws of war are and have been evolving, and there's plenty of evidence in recorded history. The first records we have come from Ancient Babylon and they have been modified and amended multiple times since. Another aspect of this development is the existence of international arbitrage with laws that also keep evolving, something that minimizes the need to go to war.

And there's more: the relationship between the state and its army changed a lot. The role, the rights and the duties of individual soldiers changed a lot. Just to give you some examples: the Lieber Code is the one created during the US Civil War -- it's perhaps the first to mention the concept of unlawful orders, exonerating the soldiers who wouldn't follow such orders. Traditionally, St. Maximilianus is described as the first conscientious objector to draft, a concept that, too, went through multiple iterations... And, really, we didn't even scratch the surface mentioning these. Military history is probably the most eventful and rich sub-field of history.