This chapter explored the implications of sharing obligations for the determination of shared responsibility, underscoring the relevance of distinguishing between two types of shared obligations throughout this process. After all, the implications of sharing obligations for responsibility relations differ depending on whether the shared obligation breached is of a divisible or indivisible character. Where a breach of an indivisible shared obligation can only result in shared responsibility for a single wrongful act – which entails that the sharing of an obligation that is indivisible always has one inevitable implication for responsibility relations – a breach of a divisible shared obligation has no such automatic implications for the responsibility of states and international organizations concerned. Instead, when a divisible shared obligation is breached, different outcomes in terms of responsibility remain possible. Depending on the circumstances in which the divisible shared obligation is breached, a breach may give rise to shared responsibility for a single wrongful act, shared responsibility for multiple wrongful acts, or may even give rise to the responsibility of only one of the states or organizations that bear the obligation (though it should be noted that, in practice, breaches of divisible shared obligations are likely to occur by multiple duty-bearers).

Throughout this book, the focus has been on developing a concept that captures the practical phenomenon of sharing international obligations, proceeding from the premise that the performance of an international obligation is not always up to only one state or international organization, and that this can have relevant legal implications. In Chapter 1, it was discussed how the idea of shared obligations has been recognized in both scholarship and practice, but comprehensive conceptualization of the notion lags behind. This was placed in the context of an overall tendency to avoid engaging with primary obligations at a more systematic level in the international law of obligations, influenced at least in part by a few fundamental choices made by the ILC during its work on the codification and progressive development of the law of treaties and the law of state responsibility. In particular, the decision to steer the focus of its state responsibility project away from primary rules that define the content of international obligations towards secondary rules1 seems to have left us with the impression that distinguishing between different categories of obligations is normally not relevant in the international law of obligations.

This chapter lays down the foundations for the argument that questions of (non-)performance raised by the practical phenomenon of sharing international obligations can be tackled by a further categorization and systematization of international obligations, since different types of shared obligations can have different legal implications. It starts by introducing the distinction between indivisible and divisible shared obligations, after which the analysis turns to the issue of how to ascertain the nature of shared obligations, as it will not always be apparent at first glance whether a particular shared obligation is to be qualified as indivisible or divisible. It will be discussed how the distinction between positive and negative obligations as well as the distinction between obligations of conduct and result can facilitate the categorization of a particular shared obligation as either divisible or indivisible. The chapter ends with some reflections on the performance of shared obligations, highlighting how both indivisible and divisible shared obligations can have relevant implications for what is expected of duty-bearers (though in different ways), necessitating a more collective approach to performance.

Throughout the years, various legal concepts have emerged that recognize that more than two states (in the form of one duty-bearer and one right-holder) may be involved when it comes to the performance of an international obligation, and that this can have relevant implications for treaty and responsibility relations. In this respect one may think of concepts such as obligations erga omnes, interdependent obligations and jus cogens. This chapter explores and analyses the way in which the international law of obligations, through these concepts, has addressed legal relations that involve more than simply one duty-bearer and one right-holder. By doing so, the chapter aims to illustrate the limits but also the B1:L9

This chapter clarifies some key terms and situates the phenomenon of sharing international obligations within the current body of international legal doctrine. The conceptualization of shared obligations takes place in the context of the international law of obligations, and it is first discussed what is meant when reference is made to this body of law. The focus then turns to the concept of shared responsibility in international law, and some preliminary reflections are offered on how the concept of shared obligations contributes to the ongoing discussion on problems of shared responsibility in legal doctrine. It is then discussed how the idea of sharing international obligations has been recognized in international legal literature and proceedings before international courts, reflecting an (at times implicit) assumption that the sharing of international obligations has relevant legal implications. Nevertheless, the notion of shared obligations remains conceptually underdeveloped. Finally, it is set out how this lack of conceptualization of shared obligations is not a result of the notion’s irrelevance but may be explained – at least in part – by a few fundamental choices that have been made during the ILC’s project of codification and progressive development of the international law of obligations.

This chapter sets out to capture the practical phenomenon of sharing international obligations by developing a concept of shared obligations. In the stipulation of this concept, the chapter draws from and engages with various scenarios in practice that involve obligations incumbent on multiple states or international organizations, including those obligations that have been designated as ‘shared’, ‘joint’ or ‘collective’ in legal literature. By expanding on three elements that characterize a shared obligation, it is argued that the main characteristic that distinguishes obligations that are shared from obligations that are not shared is the existence of a connection between the bearers of an obligation in the performance of that obligation. It is this particular relationship between the bearers of a shared obligation that gives rise to questions regarding performance and international responsibility: who is bound to do what and, subsequently, who can be held responsible for what in case of a breach? All in all, qualifying a particular obligation as ‘shared’ brings such questions to the forefront, and thereby constitutes the first step towards addressing them.

The analysis in this chapter proceeds from the assumption that shared responsibility for a breach of a shared obligation has been established and focuses on the content of that responsibility. In the case that secondary obligations of reparation or cessation arise, what can be claimed from which responsible actor, and to what extent is this influenced by the fact that the underlying primary obligation is shared? And if injured parties wish to effectuate these secondary obligations through international adjudication, are there any obstacles to the claiming of cessation and reparation? It will ultimately be argued that conceiving of the content of shared responsibility as consisting of shared obligations incumbent on all responsible actors can help untangle which of the responsible states or international organizations is bound to do what in terms of cessation and reparation. In this context, the distinction between divisible and indivisible shared obligations has an important role to play. After all, determining whether responsible actors are each bound only to their own share of cessation or reparation or whether, alternatively, all of them become bound to achieve a common result has direct implications for what can be claimed from whom.