The answer is: The Law of Land Warfare, Uniform Code of Military Justice, and Code of Conduct.
The answer may vary from country to country but on an international level the United Nations Charter also provides code of conducts on how modern armies should function in times of war.
These are important in order to provide a moral structure to an Army which can become, as evidenced in history, quite ruthless.
The 13th, 14th, and 15th amendments were important additions to the constitution because they were created to give equality and aimed mainly towards African-Americans and slaves from he civil war
They attacked the axis powers in Germany, Russia and many other opposing sides of the war.
