As the first exclusively South American cooperation scheme and the new centerpiece of post-hegemonic regionalism in the region, the foundation of the Union of South American Nations (UNASUR) has recently attracted scholarly attention. While this event has been studied by means of descriptive narratives and frequently from the angle of post-hegemonic approaches, this paper draws on neofunctionalism, intergovernmentalism, the regional powers approach, and the security community approach to explain why UNASUR was founded. It thereby offers a systematic and theoretically informed account of the foundation of UNASUR as a particular phenomenon and, by extension, of the status quo of regionalism in South America. Specifically, I examine context conditions, such as the power shift in favor of Brazil, the anti-neoliberal social paradigm, and the decline of hemispheric cooperation; the interests of Brazil, Venezuela, and Argentina, and the region-wide shared interest in stable peace and energy integration; and finally, the main actors initiating and driving the establishment of UNASUR, namely the Brazilian and Venezuelan government. I conclude that the security community approach is best able to explain the founding of UNASUR.