Branches of international law arise mainly as a result of the following factors: 1) complications of international legal consciousness (international human rightslaw, international security law, international environmental law, international criminal law, etc.) and 2) technological progress (international air, space, nuclear law, relevant innovations in international maritime law, etc.). Under the influence of the same factors, new institutions are formed within existing branches (responsibility for international crimes, prohibitions of genocide, prohibitions of apartheid, responsibility for environmental pollution, instant custom, etc.). The article examines the mechanism of formation of the international humanitarian law, the law of armed conflicts, international security law, international human rights law, and international criminal law. It is claimed that they are based on a traditional section of pre-classical international law - the law of war. The first attempts to regulate the conduct of armed conflicts formed the basis for the creation of these branches. The first scientific systematization of international law was one proposed by Hugo Grotius. The scholar proposed to divide the system of this law into two interrelated components - the law of war and the law of peace. Grotian systematization formed the basis of further scientific systematics of international law in the 17th-19th centuries. The Hague Peace Conferences changed the traditional view of the division of international law into the law of war and peace. The Hague peace conventions influenced on formation of the international humanitarian law. The 20th century became a period of active formation of new branches of international law within the framework of the process of "codification and progressive development of international law" under the auspices of the United Nations. So the traditional law of war gave birth to several branches of modern international law.