This chapter examines collective self-defence treaty arrangements. It engages with a diverse range of examples of the collective self-defence treaties (or treaties that contain collective self-defence aspects) that have emerged since 1945 to draw out common themes as to the nature, process, and role of such arrangements, as well as to establish notable variations. The aim is to contribute an overall picture of collective self-defence today specifically in the context of treaty relationships. The chapter argues that such relationships inevitably impose only weak obligations on their parties to defend each other and also can cause notable issues related to overlapping memberships, bureaucracy, and antagonism amongst members (amongst other difficulties). Equally, these arrangements – of which there are now hundreds – are concluded for good reason(s). They provide a range of benefits, especially in terms of their deterrent effect.

This chapter considers the interaction of the Security Council with other international organizations in relation to the use of force and the role that international organizations may play in implementing Council authorizations to use force. This is an area where the Security Council, the international organizations concerned, and member states have shown great flexibility, with the provisions of the UN Charter (both Chapter VII and Chapter VIII) and of the constituent instruments of the regional and other organizations being developed through extensive practice. A difficult question, however, arises where member states grant regional organizations the authority to carry out such interventions without the target state’s consent to the specific action and without Council authorization, in particualr in light of Article 4(h) of the Constitutive Act of the AU. The chapter concludes that it cannot be said that Article 4(h) of the AU Constitutive Act or similar provisions allow for the use of force without Security Council authorization. The Council remains the body vested by member states with the power to authorize ‘enforcement action’.