For practitioners of international relations, the challenge is to on one hand strike a balance between establishing a (more) predictable international system and attaining some consensus on the need to extend participation and to innovate, and on the other hand recognize, as complexity theory in social sciences suggests, the need to rationalize the behavior. Therefore, we need to seek mechanisms to ensure efficiency, predictability and mitigation failures of the international security system (ISS), which is not only sizable, but has immense inertia and often operates at the edge of chaos. The ISS is therefore challenging for any plural leadership to govern with adequate legitimacy, as it must deal with sovereign actors possessing two distinctive features: (1) They all have, in principle, equal sovereign and legal status, and (2) they are perceived to have different geopolitical power statuses. To further complicate the system, there is an additional third characteristic—each sovereign actor’s political culture is significantly different from its peers’; some are prone to exclusion, conflict and war and are less inclined to mutual restraint and cooperation, while others may like to avert conflict (and its escalation) as well as disorder, be sympathetic to attaining mutual benefits and be willing to be accommodating, though often through playing political games that still stem from ploys to advance their own collations of interests. Consequently, the ISS as a large system can be chaotic, wherein chaos is “conceptualized as extremely complex information, rather than as an absence of order” (Hayles, 1991). To reform the ISS implies a substantial redesign of the United Nations Security Council (UNSC). In 1992, the UNGA put forward an open-ended working group to review the equity of representation on the UNSC. In 2008, recognizing the importance of this matter and the lack of apparent progress, the UNGA began intergovernmental negotiations (IGNs) revolving around equitable representation and increasing the UNSC’s membership. In 2022 (GA/12435, 12 July), the UNGA adopted the Oral Decision to Continue Intergovernmental Negotiations on Security Council Reforms, in which statements from different states revealed a scenario of division, sectarianism and fragility. These concerns have prompted this research to adopt social sciences’ complexity theory to study how the ISS as a higher-level system makes global decisions in a framework of chaos. Complexity theory therefore lends “a new lens… for understanding the international system and its change, given that it fits quite well into the features of complex adaptive systems” (Tomé et. al., 2019, p. 13). This study also combines structural realism (which considers the power status of agents), post-structuralism (which considers the formation of meaning producing structures (Campbell, 2013)) and the Copenhagen School’s theory on securitization. Consequently, the study will be driven by an inquiry to understand if the ISS can be (re)designed in order to ensure legitimacy and effectiveness, and to make sure if one or a few elements fail, the impact on its overall performance can still be mitigated. In this vein of reasoning, the main focus of this research is to reform the UNSC as the institution that governs the ISS, which is currently one of the most significant topics on the international agenda. This chapter is organized as follows: Sect. 1 introduces and discusses some possible takeaway from complexity theory; Sect. 2 advances a possible definition of the international order, and outlines current discussions on reforming the UNSC; Sect. 3 formulates a framework for redesigning the UNSC based on complexity theory; and Sect. 4 draws conclusions as well as offers avenues for further research.