What documents give structure to the army's moral standards?

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 Law of Land Warfare, Uniform Code of Military Justice, and Code of Conduct.

Explanation:

These are the documents that give structure to the army's moral standards. These are important documents as they ensure that the army does not become ruthless, overly cruel or self-interested. It also reminds the army of its duty to protect citizens. In a wider sense, soldiers also take moral cues from the Constitution and the Declaration of Independence.