"Among civilized nations the conduct of war is regulated by certain well-established rules known as the rules or laws of war. These rules cover and regulate warfare both on land and sea. Those which pertain particularly to war on land are called the rules of land warfare. It is the latter with which this manual is concerned." --Article 1, Rules of Land Warfare

The current laws of war are based on a number of historical codes that regulated the conditions for war and the conduct of warring parties. One of those codes related to armed land forces of the US was Rules of Land Warfare, which was first released in 1914. This edition was updated in 1940.

Some other related publications by Cosimo Reports are: Instructions for the Government of Armies of the United States in the Field--The Lieber Code (1863), The Law and Usages of War at Sea--A Naval War Code (1900), Small Wars Manual (1940), The United States Department of Defense Law of War Manual (2016), and others.