International humanitarian law (‘IHL’), also known as the law of armed conflict (‘LOAC’), is a branch of international law that regulates the behaviour of participants in armed conflicts and encompasses rules on the military targeting of persons and objects, the means and methods of warfare, the protection of persons and objects, and the implementation and enforcement of the law. International humanitarian law is also one of the most highly codified fields of international law, with numerous treaties adopted over the last 150 years; many of these have also attained customary status. In developing IHL, states have recognised that, while the ultimate aim of society should be to strive to prevent war, wars will nonetheless occur, and that while ‘war will always constitute suffering and personal tragedy … rules of warfare are intended to prevent unnecessary suffering that yields little or no military advantage’. This balance between humanitarian aims and military objectives serves as the basic underpinning of all the modern laws of armed conflict.

To explain the law of state responsibility and diplomatic protection, it helps to distinguish between primary rules and secondary rules. The primary rules of international law provide that certain acts or omissions are unlawful – for example, the law on the use of force would be considered primary rules of international law, a breach of which would be an internationally wrongful act. When those primary rules are breached, it is the secondary rules – the law of state responsibility – that come into play, to determine inter alia the consequences of that initial wrongful act, whether the wrongful act was committed by a state (thereby entailing that state’s responsibility), and what action the ‘wronged state’ may take in reply. The rules on state responsibility cover wrongful acts committed against another state, as well as certain wrongful acts committed against nationals of the state, including corporations; the law of diplomatic protection solely concerns how a state may raise a claim against another state for a wrong committed against one of its nationals, rather than against the state itself.

The starting point for discussion and analysis of the sources of international law is almost invariably art 38 of the Statute of the International Court of Justice (‘ICJ Statute’), the International Court of Justice being the primary judicial organ of the United Nations. Article 38 lists the sources of international law as comprising treaties, custom, general principles of law, and – as subsidiary means for determining the law – judicial decisions and academic writing. However, in the 75 years since the adoption of the ICJ Statute, newer sources of legal obligation have emerged for the international community. These often involve non-state and intergovernmental actors in their creation. This chapter explores both the traditional and newer sources of international law and assesses how they are adopted and created.