This chapter introduces the jus ad bellum: the rules of law determining when states may resort to war or, more broadly, the use of armed force. These rules must be distinguished from the jus in bello: these are the rules of law that apply in armed conflict (known as international humanitarian law). In order to put the current jus ad bellum rules into perspective, the chapter begins by introducing the concept of collective security and demonstrating how this was applied during the League of Nations era. The following sections set out the relevant rules of the UN Charter on the prevention and regulation of recourse to the use of force, with a particular focus on the prohibition on the threat or use of force. The chapter also discusses the collective use of force, meaning the use of force authorized by the Security Council, and the unilateral use of force in self-defense. Finally, the chapter examines whether new exceptions to the prohibition of the use of force are emerging -- in particular, humanitarian intervention and the responsibility to protect.

This chapter focuses on the rules set out in the 1969 Vienna Convention on the Law of Treaties (VCLT). The chapter begins with the concept of a treaty, before discussing treatymaking, with a particular focus on the conclusion of treaties, their entry into force, and reservations to treaties. The chapter then delves into how treaties operate -- namely, their scope of application and their interpretation. Finally, this chapter looks at the invalidity, suspension, and termination of treaties.

International criminal law is the branch of public international law under which individuals may be held criminally responsible for the offenses of genocide, crimes against humanity, war crimes, and the crime of aggression. International criminal law seeks to prevent impunity by holding accountable those individuals who are responsible for serious violations of international criminal law. This chapter begins with the history of international criminal law, starting with the aftermath of the First and Second World Wars, and ending with the creation of a spate of international criminal courts and tribunals in recent decades. The chapter then covers substantive aspects of international criminal law, namely the crime of genocide, crimes against humanity, war crimes, and the crime of aggression. Finally, the chapter covers key procedural aspects of international criminal law, including the jurisdiction of international courts and tribunals, the admissibility of cases, modes of liability, and immunities. The International Criminal Court (ICC) will form a focal point in this chapter.

International criminal law is the branch of public international law under which individuals may be held criminally responsible for the offenses of genocide, crimes against humanity, war crimes, and the crime of aggression. International criminal law seeks to prevent impunity by holding accountable those individuals who are responsible for serious violations of international criminal law. This chapter begins with the history of international criminal law, starting with the aftermath of the First and Second World Wars, and ending with the creation of a spate of international criminal courts and tribunals in recent decades. The chapter then covers substantive aspects of international criminal law, namely the crime of genocide, crimes against humanity, war crimes, and the crime of aggression. Finally, the chapter covers key procedural aspects of international criminal law, including the jurisdiction of international courts and tribunals, the admissibility of cases, modes of liability, and immunities. The International Criminal Court (ICC) will form a focal point in this chapter.

This chapter sketches the development of international human rights law. The legal position of individuals was perceived by states as a domestic affair of the sovereign state, which could effectively treat its citizens as it pleased. It was not until after the Second World War that this fundamentally changed and international law began to grant individuals rights to protect them from the state. The chapter further presents the main categories or generations of human rights and discusses their key characteristics. Human rights are generally organized in three categories or generations: (a) civil and political rights; (b) economic, social, and cultural rights; and (c) collective rights. This chapter will subsequently turn to several matters concerning the scope of human rights norms, including their addressees, their territorial scope of application, and the circumstances under which human rights may be restricted by the state. Finally, this chapter will explain how human rights are monitored and enforced under international law.

IHL has historically been divided into two main branches, consisting of rules that regulate the “means and methods of warfare”and the rules that deal with the “protection of persons and projects.” The rules governing the means and methods of warfare are known as “Hague Law” due to the fact that the main treaties governing this field of law were, for a long time, the 1899 and 1907 Hague Conventions and the annexed Hague Regulations. The provisions dealing with the protection of persons and objects hors de combat (“out of combat”) are known as “Geneva Law,” as these rules can be found in the Four Geneva Conventions of 1949. The two Additional Protocols of 1997 cover both norms concerning the means and methods of warfare, and those protecting individuals. This chapter begins with the foundations and the history of IHL, before discussing the scope of application of IHL and the law governing the conduct of hostilities, namely the means and methods of warfare. The final sections discuss the law governing the protection of persons during armed conflict, and the implementation and enforcement of IHL.

The rules of state responsibility are set out in the International Law Commission’s (ILC) Articles on the Responsibility of States for Internationally Wrongful Acts. This chapter introduces the basic features of the ILC’s Articles on State Responsibility, beginning with an explanation of what constitutes an internationally wrongful act. The following section discusses the circumstances precluding wrongfulness that may be invoked by states seeking to avoid responsibility for an internationally wrongful act. The chapter then covers the aftermath of an internationally wrongful act, which can involve legal consequences, such as reparations, as well as countermeasures. This chapter focuses specifically on the responsibility of states, rather than the responsibility of international organizations or individuals.

After a sketch of the history of the law of the sea and the traditional freedoms of the sea, this chapter discusses the various efforts to codify the law of the sea during the twentieth century, which culminated in the adoption of UNCLOS in 1982. Subsequently, the chapter examines the legal regimes governing the various maritime zones, as well as two international areas: the high seas and the Area. The chapter then takes up thematic issues in the law of the sea, namely the delimitation of maritime boundaries, the protection of the marine environment, the special interests of developing countries, and the system for the settlement of law of the sea disputes. The chapter concludes by noting that, despite the relatively comprehensive scope of UNCLOS, a number of new challenges have arisen with respect to the law of the sea, especially as a result of human-driven climate change.

Each state enjoys full sovereignty over its territory and therefore has, in principle, full jurisdiction over events and persons within its territory. The term “jurisdiction” is often associated with the power of a court to hear a case, but judicial or adjudicative jurisdiction is just one of three manifestations of jurisdiction. The exercise of jurisdiction is also manifested in the authority of states to prescribe rules (legislative or prescriptive jurisdiction) and the power of states to enforce rules (enforcement jurisdiction). Although the power of states to exercise jurisdiction follows from the principle of sovereignty, this power is not unlimited. Each state has to respect the personality and sovereign equality of other states, and overstepping this boundary entails a violation of public international law. This limitation is most pertinent when a state exercises jurisdiction outside its own territory, for example when it enforces domestic legislation outside of its territory or when it extends the application of its domestic laws to people, property, or events outside its own territory. This chapter discusses the scope of the jurisdiction of states when states exercise enforcement, prescriptive, and adjudicative jurisdiction in relation to persons, property, and acts outside their own territory.

This chapter focuses on the rules set out in the 1969 Vienna Convention on the Law of Treaties (VCLT). The chapter begins with the concept of a treaty, before discussing treatymaking, with a particular focus on the conclusion of treaties, their entry into force, and reservations to treaties. The chapter then delves into how treaties operate -- namely, their scope of application and their interpretation. Finally, this chapter looks at the invalidity, suspension, and termination of treaties.

International organizations perform activities in areas in which states can no longer operate effectively in isolation, and in which there is a common interest in cooperation within a permanent international framework. This chapter will examine international organizations primarily from a legal perspective. The chapter aims to present a general overview of the law of international organizations. It discusses the legal status, privileges, and immunities of international organizations. The chapter further deals with membership issues, powers, and institutional structures. The chapter also looks at decisions of international organizations: the way in which they are taken and the different types of decisions. The chapter will briefly examine the finances of international organizations. There has been an exponential increase in activities of international organizations over the years. Not all of these activities have been successful, however, and there have been failures and wrongdoings. In recent years, a much-debated issue is the extent to which international organizations and/or their members may be held responsible for such failures and wrongdoings.