This chapter explores the foundations of due diligence under international law. Due diligence emerged in the international practice of the nineteenth century concerning diplomatic protection and the security of states, and developed as a notion linked to the responsibility of states in connection with acts of private individuals. For a long time, due diligence was conceived as a concept pertaining to the realm of international responsibility and it was primarily associated with the measure of a state organ’s fault. The chapter illustrates how, during the twentieth century, due diligence migrated from the realm of secondary rules to primary rules. The chapter clarifies the relationship between due diligence and overlapping concepts, like international liability and the notion of general principles of international law. It is argued that due diligence should be construed as an identifier for a typology of international obligations, something that provides meaning and rationale to them. The chapter concludes by clarifying the difference between due diligence as a ‘qualifier’ for primary rules of states, and due diligence as a ‘process’ linked to the activities of non-state actors.

International liability and international criminal law are presented as alternatives to the law of state responsibility. Both regimes have developed out of real-life incidents to which state responsibility has not offered a sufficient enough solution. With their respective focus on adequate compensation and the desire to penalize perpetrators for the most serious violations of international law, they represent qualitatively different approaches to state responsibility. A more limited test of functionality is conducted in order to analyze whether the practical utility of these two regimes is higher than concerning the law of state responsibility. Three criteria that are crucial to a well-functioning and practical responsibility regime are explored: social control, collectivity, and signalling effect. The thematic evaluation of the two responsibility regimes shows that in some respects these particularized regimes fare better than state responsibility while also suffering from problems internal to their particular regimes. The core of the matter is that both regimes have their own rationale from which they do not purport to slide.

The concluding chapter submits that state responsibility, which forms part of the ground rules of international law, is not equally important across the broad spectrum of international obligations. The functionality of the general rules of state responsibility varies as a result of the multiple purposes state responsibility is expected to fulfil: the generality of the law stresses its system-building function rather than practical usefulness. As a result, other responsibility constructions have emerged next to the law of state responsibility seeking to fill voids left by the law of state responsibility. The importance of particularized solutions are emphasized, as well as calls for the acceptance of the fragmentation of international responsibility. It is submitted that state responsibility no longer enjoys a unique position in international law. Instead, several avenues of responsibility all embody the core idea that legal consequences will follow from breaches of the law.