Historian Geoffrey Best looked at the time period between 1856 and 1909 and called it the "epoch of highest repute" for the development of the law of war. During this period, written rules or laws established by international conventions replaced a system based on religion, chivalry and custom.
Here are some sites that will help you find out more about the law of war as it developed prior to World War I.
The Laws of War (Avalon Project, Lillian Goldman Law Library, Yale Law School): Online documents relating to the development and creation of the international laws of war.
The Hague Peace Conferences (1899 and 1907): A brief explanation of the Hague Conferences which are a sequence of international treaties and declarations negotiated at two international peace conventions at the Hague in the Netherlands. The first was held in 1899, the second in 1907 and were among the first official proclamations of the laws of war and war crimes. A third conference was planned for 1914 and later rescheduled for 1915, however it never took place because of World War I.
WWI Conventions and Treaties (Brigham Young University): Includes all of the treaties and conventions between 1856 and 1928.