This document is the updated version of the Department of Defense Law of War Manual. "This manual is a Department of Defense (DoD)-wide resource for DoD personnel - including commanders, legal practitioners, and other military and civilian personnel - on the law of war. This manual has many distinguished antecedents that have provided important guidance to the U.S. Armed Forces. For example, General Order No. 100, the Instructions for the Government of Armies of the United States in the Field, commonly known as the Lieber Code, was prepared by Professor Francis Lieber and approved by President Abraham Lincoln during the Civil War in 1863. A similar code related to naval warfare titled The Law and Usages of War at Sea: A Naval War Code was prepared by then-Captain Charles H. Stockton and approved by President William McKinley in 1900. The War Department published instructions for the armed land forces of the United States in a 1914 manual titled Rules of Land Warfare, which was updated in 1917, 1934, and 1940."
United States, Department of Defense: http://www.defense.gov/