The first and the main source of international humanitarian law is to be found in treaties. History tells that rules of International humanitarian Law, particularly rules on the treatment and exchange of prisoners and wounded, have since long been laid down in bilateral treaties. The systematic codification and progressive development of this branch in general multilateral treaty also started in the midst of the 19^th century, which is relatively early as compared with other branches of international law.

A salient feature of the treaties of international humanitarian law is that most often a new set of treaties are supplemented or replaced with more details earlier ones after major wars taking into account new technological or military developments. Treaties of international humanitarian law have therefore been accused of being “one war behind reality”. This is however true for all law and it is only rarely has it been possible to regulate or even to outlaw a new means or method of warfare before it has been applied. 

Today, international humanitarian law is not only one of the most codified branches of international law but its relatively few instruments are also rather well coordinated with each other.

Of all the treaties signed so far, the four Geneva Conventions of 12 August 1949 for the protection of the victims of war are making up the main sources of international humanitarian law. The first of these conventions is Convention for the Ameliorations of the Conditions of the Wounded and sick in Armed Forces in the Field. The second Geneva Convention is Convention for the Amelioration of the Condition of Wounded, Sick and Shipwrecked Members of Armed Forces at Sea; and the third one is Convention relative to the Treatment of Prisoners of War; and fourthly there is Convention relative to the Protection of Civilian Person in Time of War.

These four Geneva Conventions have also been supplemented with the two Additional Protocols of 8 June 1977. One of which, Protocol I, is Protocol Additional to the Geneva Conventions of 12 August 1949, and relating to the Protection of Victims of International Armed Conflicts; and Protocol II is Protocol Additional to Geneva Conventions of 12 August 1949, and relating to the Protection of Victims of Non-International Armed Conflicts.

These treaties have the great advantage of putting their rules relatively beyond doubt and controversy, “in black and white”, ready to be applied by a soldier without needing first to make a doctoral research on practice. They, furthermore, legitimize their rules for the majority of “new states” which are able to influence them in the elaboration process and which can more easily agree to be bound by them in their frequently voluntarist approach.

The disadvantages of these treaties, as of all treaty law, are that they are technically unable to have a general effect-automatically to bind all states. Fortunately, most of the treaties of international humanitarian law are considered today among the most universally accepted treaties and only few states are not bound. 

It has also been provided that however important the treaty rules of international humanitarian law may be even if they constitute obligations erga omnes, belong to jus cogens and if their respect is not subject to reciprocity- as treaty law they are only binding on states part to those treaties and, as far as international armed conflicts are concerned, only in their relation with other states parties to those treaties. The general law of treaties governs the conclusion, entry into force, reservations, application, interpretation, amendment, modification of international humanitarian law treaties and even their denunciation, which however, only takes effect after the end of an armed conflict in which the denunciating state is involved. The main exception to the general rules of the law of treaties for international humanitarian law treaty is provided by that same law of treaties; Once an international humanitarian law has become binding for a state, even a substantial breach of its provisions by another state, including by its enemy in an international armed conflict, does not permit the termination or suspension of the operation of that treaty as a consequence of that breach.

Although international humanitarian law is a branch widely codified in widely accepted multilateral conventions, customary rules remain important to protect victims on issues not covered by treaties, when non-parties to a treaty are involved in a conflict, where reservations have been made against the treaty rules and also because of the fact that international criminal tribunals prefer to apply customary rules, and because in some legal systems only customary rules are directly applicable in domestic law. Given the time consuming nature and other difficulties of treaty-making in an international society with more than 190 members and the rapidly evolving needs of war victims for protection against new technological and other inhumane phenomena, the importance of custom, redefined or not, may even increase in this field in the future. This, therefore, indicates the fact that customary law comes to be another source of international humanitarian law.

This, however, doesn’t mean that there aren’t any difficulties in defining a certain practice in terms of whether it is a customary rule or not. Those who follow a traditional theory of customary law and consider it to stem from the actual behavior of states in conformity with an alleged norm face particular difficulties in the field of international humanitarian law. First, for most rules this approach would limit practice to that of belligerents. And this comprises a few subjects whose practice is difficult to qualify as “general” and even more as “accepted as law.” Second, the actual practice of belligerents is difficult to identify, particularly as it often consists of omissions. There are also additional difficulties, e.g., war propaganda manipulates truth and secrecy makes it impossible to know which objectives were targeted and whether their destruction was deliberate. Finally, states are responsible for the behavior of individual soldiers even if the latter did not act in conformity with their instructions, but this does not imply that such behavior is also state practice constitutive of customary law. It is, therefore, particularly difficult to determine which acts of soldiers count as state practice. 

Other factors must, therefore, also be considered when assessing whether or not a rule belongs to customary law: whether qualified as practice lato sensu or as evidence for opinio iuris, statements of belligerents, including accusations against the enemy of violations of international humanitarian law and justifications for their own behavior.  

To identify “general” practice, statements of third states on the behavior of belligerents and on a claimed norm in diplomatic fora have to be similarly considered. Military manuals are even more important, because they contain instructions by states restraining their soldiers’ actions, which are somehow “statements against interest.” Too few States, generally Western States, have, however, sophisticated manuals available to the public to consider their contents as evidence for “general” practice in the contemporary international community.

It is also logically argued and even said to be totally uncontroversial that most, but clearly not all, rules of the two 1977 Additional Protocols today provide a formula for parallel rules of customary international law. Taking an overall view of all practice it can, for instance be found that a rule of the two 1977 Additional Protocols corresponds today to customary law binding on all states and belligerents, because it codified previously existing general international law, or because it translated a previously existing practice into a rule, because it combined, interpreted, or specified existing principles or rules, or because it concluded the development of a rule of customary international law or finally because it was a catalyst for the creation of a rule or of customary international law  through subsequent practice and multiple consent of states to be bound by the treaty.

Custom, however, has also very serious disadvantages as a source of international law. It is very difficult to base uniform application of the law, military instruction and the repression of breaches on custom which by definition is in constant evolution, is difficult to formulate, and is always subject to controversy. The codification of international humanitarian law began 150 years ago precisely because the international community found the actual practice of belligerents unacceptable, while custom is, despite all modern theories, also based on the actual practice of belligerents.