The modern law of war is made up fr

The modern law of war is made up from three principal sources:[1]

Lawmaking treaties (or conventions) — see § International treaties on the laws of war below.
Custom. Not all the law of war derives from or has been incorporated in such treaties, which can refer to the continuing importance of customary law as articulated by the Martens Clause. Such customary international law is established by the general practice of nations together with their acceptance that such practice is required by law.
General Principles. "Certain fundamental principles provide basic guidance. For instance, the principles of distinction, proportionality, and necessity, all of which are part of customary international law, always apply to the use of armed force".[1]
Positive international humanitarian law consists of treaties (international agreements) which directly affect the laws of war by binding consenting nations and achieving widespread consent.

The opposite of positive laws of war is customary laws of war,[1] many of which were explored at the Nuremberg War Trials. These laws define both the permissive rights of states as well as prohibitions on their conduct when dealing with irregular forces and non-signatories.

The Treaty of Armistice and Regularization of War signed in the Venezuelan city of Trujillo in November 25 and 26 1820 between the president of the Republic of Colombia, Simon Bolivar and the Chief of the Military Forces of the Spanish Kingdom, Pablo Morillo, is the precursor of the International Humanitarian Law.[8] The Lieber Code, promulgated by the Union during the American Civil War, was critical in the development of the laws of land warfare.[9] Historian Geoffrey Best called the period from 1856 to 1909 the law of war’s "epoch of highest repute."[10] The defining aspect of this period was the establishment, by states, of a positive legal or legislative foundation (i.e., written) superseding a regime based primarily on religion, chivalry, and customs.[11] It is during this "modern" era that the international conference became the forum for debate and agreement between states and the "multilateral treaty" served as the positive mechanism for codification.

In addition, the Nuremberg War Trial judgment on "The Law Relating to War Crimes and Crimes Against Humanity"[12] held, under the guidelines Nuremberg Principles, that treaties like the Hague Convention of 1907, having been widely accepted by "all civilised nations" for about half a century, were by then part of the customary laws of war and binding on all parties whether the party was a signatory to the specific treaty or not.

Interpretations of international humanitarian law change over time and this also affects the laws of war. For example, Carla Del Ponte, the chief prosecutor for the International Criminal Tribunal for the former Yugoslavia pointed out in 2001 that although there is no specific treaty ban on the use of depleted uranium projectiles, there is a developing scientific debate and concern expressed regarding the effect of the use of such projectiles and it is possible that, in future, there may be a consensus view in international legal circles that use of such projectiles violates general principles of the law applicable to use of weapons in armed conflict.[13] This is because in the future it may be the consensus view that depleted uranium projectiles breach one or more of the following treaties: The Universal Declaration of Human Rights; the Charter of the United Nations; the Genocide Convention; the United Nations Convention Against Torture; the Geneva Conventions including Protocol I; the Convention on Conventional Weapons of 1980; the Chemical Weapons Convention; and the Convention on the Physical Protection of Nuclear Material.