Given Brazil's regional prevalence, its low, late and soft investment in regional security governance appears puzzling. We approach the puzzle through an analysis of contextual features, institutional overlap and policy networks, especially regarding nuclear energy and the environment. Our findings show that Brazil's behavior is explained by a combination of low regional risks, scarce domestic resources, a legalistic regional culture of dispute settlement, and transgovernmental networks that substitute for formal interstate cooperation and deep regional institutions.Keywords: Brazilian foreign policy; Latin American regionalism; Regional Security Governance; Regional Powers; Policy Networks. Introduction B razil is the largest Latin American country and represents half of South America, whether regarding area, population, the economy or overall national capabilities. Its rise has led many to expect it to become a regional leader, as both its defensive and offensive foreign interests spill over onto its neighbors (Arnson and Sotero 2009;Bethell 2010;Stuenkel 2013a;Trinkunas 2014). Yet, this expectation has not come to pass (Malamud 2011;Schirm 2010). Neither has Brazil shown the ambition or capacity to impose itself, nor have its neighbors felt the need to balance its power (Gardini and Almeida 2016;Schenoni 2015). Still, regional exchanges have increased substantially over the last three decades. In this article, we focus on the most crucial dimension of these interactions, namely security, in order to understand Brazil's role in building regional governance.Brazil's home region has two central peculiarities: first, its boundaries are fuzzy, second, its inner core has been characterized Copyright:• This is an open-access article distributed under the terms of a Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided that the original author and source are credited.• Este é um artigo publicado em acesso aberto e distribuído sob os termos da by a long period of interstate peace. The region's boundaries are fuzzy because Brazil has moved them over time: from encompassing all of the Western Hemisphere, to just Latin America, and finally at present to simply comprise of the South American subcontinent (Malamud and Gardini 2012; Spektor 2010). More significantly, its inner core -namely South America -has been peaceful, given the absence of interstate war between major powers since 1883, and altogether since 1942 (Kacowicz 2005; Mares 2001). The enduring peace of the region has led to high politics being conducted through diplomatic rather than military means; its fuzziness means that region-building remains an instrument at the service of national strategies rather than a regional goal per se, as no intrinsic region exists.Given the historically low level of intra-regional exchanges, regional public goods have usually been negatively defined, mostly as the avoidance of harmful externalities. No country in the region has been capable of either...