While different nations have different laws of war, this society has laws of war similar to those followed by the United States Armed Forces; how would a militaristic society develop that saw its laws of war as sacred?
Technology level is similar to our 21st century military technology (with some advanced technology like lasers but they're like experimental prototypes).
Also this society has been around for many years.
This society sees the usage of chemical, biological, and nuclear weapons as paramount to war crimes (the usage of nuclear weapons is only to be used as a last and final result and when all other options are exhausted).
Prisoners of war are treated with respect and the usage of enhanced interrogation (aka torture) is refrained from (except in the most dire of circumstances).
Deliberate killing of civilians without cause is punishable by the death penalty. This also applies to the enemy. Enemy commanders who have committed this act are shot on sight. Children are never to be harmed (if enemy commanders have forced children to act as soldiers, then said enemy commanders are to be shot on sight). Enemy soldiers who have conducted said mistreatment of civilians will be investigated and if found guilty by their own free will shall be executed. Those who were forced to harm civilians against their will shall be imprisoned and treated as 'forced combatants' and be reconditioned if necessary.
Assisting civilians and even wounded enemy combatants is praised and those who performed such deeds are highly regarded by society.
A clear declaration of war must be given.
Looting and plundering of historical areas is considered a war crime and usually results in court-martial and extensive jail time. Destruction of historical areas and items faces the same punishment (although execution will be added if civilians are harmed during the destruction of said historical items and areas).