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 christians fought no one in particular, as they fought everyone. including but not limited to <span>germans, bulgarians, byzantines, seljuks, arabs, jews, each other, etc.</span>
<span>Nixon's scandal shook the public's trust in government</span>
