There most certainly are rules that are followed. If they're not followed bad things, like Nuremberg happen to you.
Only if you lose. Look at what the Russians did to German POWS. Look at what the Russians did to their own returning POWs. How many Russians were prosecuted for war crimes at the end of WWII.
Hell, Stalin refused to let civilians flee Stalingrad and forced them to endure cold, starvation, and war for selfish pride. And that was his own people.
The victors decide war crimes. We let our soldiers die by taking away tools that may save their lives. At the same time, we deal with and enemy that targets women and childeren, and brag about their deaths. We prosecute soldiers for war crimes for killing the enemy, and defending themselves. Then we make excuses for enemy beheading reporters and soldiers. We just make ourselves look like mommy parts. And that is exactly how our enemy sees us.
Look at our own behaviour in WWII, to know. We burned Tokyo to the ground with firebombs. We burned Hamburg to the ground with firebombs. We destroyed every other industrialized city in Germany and Japan civilians be damned. In fact, we acknowledge it was a large tool to break the will of the enemy to fight. Now we prosecute our soldiers for taking too many head shots, or returning fire when the enemy is using human shields. It is no wonder the enemy won't give up. They have seen we have lost our will to fight.
How many of us all would toss any and all rules out the window immediately to save a member of our families?
There is no higher moral ground in war. There is winning and losing. The moral ground is defined after the war by the victor.