Introduction The United Nations (UN) was founded in 1945 to ‘save succeeding generations from the scourge of war’ and is the paramount modern day embodiment of collective security. The UNs record in ‘reducing threats to international security’ has been less than stellar; the 2004 UN High-level Panel Report on Threats, Challenges and Change itself conceded that there have been ‘major failures and shortcomings’. However, in spite of its problems and failures, the UN remains the most sublime embodiment of the human aspiration for a better future and, arguably, our best chance of achieving this aspiration. It is with this in mind that negotiations on fundamental UN reform are underway. The aim of this essay is to contribute to the discourse on UN reform by discussing the key challenges facing collective security as a mechanism of international order in the 21st century. Given this practical slant, the essay eschews a metaphysical discussion of the challenges of collective security (e.g., lack of universal belief in ‘indivisibility of peace’ , free-ridership and collective-action problems) . Instead, it will focus on the challenges, complexities and tensions that the UN has and will face in the 21st century. The first half of the essay discusses challenges to ‘traditional’ collective security, wherein ‘an attack against one, is an attack against all’. (Here, collective security is defined as a group of states agreeing not to attack one another, and to defend each other against an attack from one of the others. ) The second half of the essay discusses the challenges arising from a more contemporary and broader definition that incorporates threats in the ‘new security agenda’. The UN: Collective Security or Balance of Power? Before proceeding further, it is crucial to highlight the thinking behind the UNs formation, which underpins subsequent discussion. While the UN embodies global collective security, it was never intended as a utopian exercise. From the onset, it was designed to balance between principle and power, idealism and realism and, crucially, collective security and balance of power. Therefore, on one hand, the UN Charter creates a rules-based global order based on principles of state sovereignty and non-interference in internal affairs; states are to refrain from the threat and use of force against other states unless in self-defense or as authorized by the Security Council (SC). On the other hand, the five permanent SC members (P5 comprising US, China, Russia, United Kingdom and France) were given greater responsibility and, correspondingly, disproportionate privileges via permanent SC membership and the ‘right to veto’. Challenges ‘We seem to have lost consensus on basic principles about what constitutes a threat to peace and security, about when the use of force is legitimate and advisable, and about who should authorize it.’ Kofi Annan, 2005 Questions Over Legitimacy and Effectiveness While the existence of the P5 and veto has enabled the UNs creation and functioning, it has also resulted in fundamental questions regarding its legitimacy and effectiveness as a mechanism of ‘traditional’ collective security. Firstly, the legitimacy of the UN to represent and act in the