Civilised Warfare

What about that statement? How can one have civilised warfare? If one country is at war with another, who or what suggests we should adhere to given rules of engagement? 

If we are to read “The Art of War” by Sun Szu, there are many rules he states which have stood the test of time. But what he does not mention in his book is the act of taking a hostage or what to do should the enemy surrender. I am aware the Japanese feel it is better to die in battle than to be captured as this saves face. Their saying is: to lose everything is not to lose face. But to lose face, is to lose everything. 

Should one treat a prisoner of war with contempt or with mercy? Just because the Geneva Convention stipulates certain acts as “war crimes” does not mean all countries will abide by those terms. How can one have “crimes” when one is at war? I don’t understand the logic. Surely, if you are at war and your enemy is within firing range, you shoot them. If they have a medic running around making your enemy better, the first person you should shoot is the medic. In my rules of engagement, this would mean that I am making my enemy weaker and less able to defend themselves if the one person who can heal them is dead, then the wounded will surely die making them less of a threat to me. 

Does anyone else subscribe to this line of thought? Let’s take it one step further: what if a child were to attack an officer under my command and the child was shot due to his act? Would this be considered a moral ground for “war crime” or would it simply be the enemy taking what actions it needs to ensure its army is not under any unnecessary threat?