This project aimed to identify and analyse the variables that explain the origin and level of effectiveness achieved by the Treaty of Tlatelolco, which created the Latin American and Caribbean Nuclear-Weapon-Free Zone (LAC-NWFZ). The Treaty was opened for signature in 1967, but only in 2002, it entered into force in the 33 states of the continent. Tlatelolco’s states parties undertook to prevent any form of possession of nuclear weapons, while Nuclear Weapon States (NWSs) pledged not to attack nuclearly the zone or breach its denuclearized status. However, presumable violations have occurred and NWSs still had interpretative declarations over the Treaty jeopardizing its compliance. Therefore, to what extent Tlatelolco has been effective to tackle nuclear threats and nuclear proliferation and what factors can explain it? To answer this inquiry, we analysed multidimensionally the nuclear capabilities and non-proliferation preferences of Mexico, Argentina and Brazil between 1947 and 1997, and the performance of the Agency for the Prohibition of Nuclear Weapons in Latin America and the Caribbean (OPANAL) in the same period. Despite the limited time-span of observation, we sketched out a contemporary reading of the past, concluding that Tlatelolco’s “old” challenges and threats persist until nowadays even though Latin American and Caribbean states parties have predominantly respected the mandate. Additionally, we identified the following: The Missile Crisis in Cuba was a relevant external inducement for the timing when Mexico and other four countries (Brazil, Bolivia, Ecuador, and Chile) launched the LAC-NWFZ proposal and a first 17-state group approved the Treaty. This event did not have a homogenous effect in the region; it proved to be irrelevant for advanced nuclear-developed countries like Argentina. Even though Brazil was part of the group of Tlatelolco’s proponents, its position changed once the military dictatorship took the power in 1964. NWSs did not thwart the negotiation process of the LAC-NWFZ Treaty, but they did influence the limits of Tlatelolco’s final mandate, especially the US. Tlatelolco resulted desirable for the US security interests and for its international purpose of building a nuclear non-proliferation regime and preserving its nuclear supremacy. The ideological coincidence between the US and Brazil and the US – Argentina most of the times did not mean or provoked automatic coincidence over nuclear and non-proliferation issues. Undoubtedly, the 1982 Malvinas/Falklands War between the UK and Argentina was a significant watershed for the relations of this latter with the US. This event heightened their mutual distrust and fuelled Argentina’s reluctance to sign Tlatelolco. The formal compliance of Tlatelolco by states-parties proved to be not enough to tackle critical situations satisfactorily. In the context of the Malvinas/ Falklands War, Argentina accused the UK of introducing a nuclear-armed submarine. In response to it, OPANAL issued a statement expressing concern about the incident but it was legally unable to take further actions. Within the limits of the Treaty’s mandate, there was (is) no clause or article about the process to follow in case of a presumable violation of the Treaty by an NWS or other external actors. It is not clear either how the 255 Security Council Resolution can be invoked in these contexts and how should it be coordinated with Tlatelolco. The disappointment of Argentina about the OPANAL’s response to the presumable UK’s introduction of nuclear weapons to the zone coupled with Brazil’s long-standing objective of expanding its influence towards Africa, led them to promote the South Atlantic Peace and Cooperation Zone (SAPCZ), which emphatically prohibited the introduction of nuclear weapons. Brazil’s and Argentina’s transition to democracy helped to solidify their bilateral nuclear cooperation and the implementation of the neoliberal model helped them to enhance their relations with the US. Argentina to offset its reluctance to adopt the NPT and Tlatelolco, gain international trust and thus, alleviate its vulnerable financial conditions prioritized their ties with emergent countries or pairs by becoming a member of the “New Delhi’s group of six”. Brazil since the 90’s, instead, was interested in underpinning its position as a “global trader” which led it to cultivate a peaceful and benevolent identity by integrating middle-power groups such as the “New Agenda Coalition” (NAC) towards complete disarmament. The flexibility of Tlatelolco’s adhesion process (stipulated in article 29), the introduction of the right to carry out PNEs (through article 18) and to build nuclear-propelled submarines (through article 5), as well as the active diplomatic collaboration of the OPANAL Secretaries-General to outline a better Safeguards Agreement between the IAEA and Argentina and Brazil eased the conditions for their denuclearization without hindering their nuclear civilian programs. Other systemic factors helped indirectly Tlatelolco’s entry into force in Argentina and Brazil in 1994. Here we can mention the new world order dominated by economic concerns and the signature of the US- Russia Strategic Arms Reduction Treaty (START I) perceived by both countries as a symptom of NWSs’ commitment to disarmament. In the case of Argentina, its indebtedness and economic crisis, and the US conditioned financial help weakened its international critical position pushing the country to adopt fully the Treaty. Elites’ new understandings about the state, sovereignty, and security also contributed to the adoption of Tlatelolco by Brazil and Argentina. Since the NPT entered into force in 1970, many of OPANAL’s monitoring and compliance tasks have been reassigned to the IAEA, and the Agency has become above all a body for channelling political and diplomatic tasks. Thanks to Tlatelolco and OPANAL, non-proliferation has become one of the few areas where Latin American and Caribbean countries have had a stable and unified position for almost half a century. The OPANAL’s proposal of developing a standing forum of NWFZs’ states-parties to galvanize collective action and undertake common initiatives, has proved the OPANAL’s embeddedness ability in the scaffolding of the international non-proliferation regime OPANAL’s constant financial difficulties were and are still an example of longstanding and non-solved problems, which has limited its activities and has unveiled to what extent states-parties were and are interested in the course of the Agency. The perceived gradual loss of importance of the nuclear threat in the ‘90s as well as the fact that the nuclear proliferation problem has been a sporadic and not a lingering preoccupation in the continent (given the inexistence of a proliferator) contributed to diminishing the significance of Tlatelolco. The incommensurability between the Treaty’s objectives and scopes and its real problem-solving capacity –particularly through OPANAL- was (and is) mainly externally configured by geopolitical circumstances where the power of NWSs is unbeatable. The persistent legacy of colonialism and other forms of foreign presence are still crucial challenges to the denuclearized status of Latin America and the Caribbean NWFZ despite concerning states have ratified both Additional Protocols. The particular interpretations by NWSs regarding the freedom to navigate on the high seas and the transit rights with radioactive material is a stumbling block from then to now for the effective compliance of Tlatelolco. Because of all the presented reasons, we concluded that the overall performance of the Treaty until the last observed period has been moderately effective.