Between 1975 and 1988, the Southern African regional system was marked by high levels of systemic conflict involving direct and indirect armed confrontation between South Africa and its neighbours. This reality sharply contrasts with the co-operative environment that has gradually formed since 1989. Following the trends towards deepening and widening the systemic comprehension of international relations, this study seeks to understand why there were changes in the pattern of co-operation-conflict in the Southern African regional system in the last 40 years. The central hypothesis is that (i) the state-building process and (ii) regional and secondary powers' foreign policy formation and execution towards the regional order are factors which have directly affected the regional pattern of co-operation-conflict. Specifically, this article studies South Africa and Angola's state-building process and foreign policy formation and execution from 1975 to 2015. The research concludes that these states produced co-operation or conflict as part of their balance of positions towards the systemic order, which is an interactional result of their state-building process (state capacity and state-society relations) and its impact on foreign policy's formation and execution (interests and security of the elites and foreign policy position and impetus towards the regional order). lations has limited capacity to understand regional politics, where agency and realpolitik calculus is as present as elsewhere in international politics (Chazan et al. 1999: 361-3).This paper seeks to contribute to overcoming these theoretical limitations through a study that aims to explain changes in the regional pattern of co-operation-conflict in Southern Africa, highlighting aspects of unit agency. The central hypothesis is that (i) the state-building process and (ii) the foreign policy formation and execution of the regional and secondary powers towards the regional order are factors which have directly affected the regional pattern of co-operation-conflict, and stand as possible causal variables. The argument is divided in two different parts. First, the changes in the pattern of co-operation-conflict were mostly due to the different positions that the regional and secondary powers sustained towards regional order (or status quo), forming what I call a balance of positions. Second, the comprehension of what caused those states to maintain or change different positions towards the status quo should be found not by means of mechanical logic situated in isolation at the structural level, but by an evaluation of foreign policy formation and execution in the context of the state-building process.In specific terms, one analyses South Africa and Angola's foreign policy execution towards the regional order, informed by their state-building process and foreign policy formation. The article is structured into three sections. Firstly, it introduces the debate on how to assess changes in regional systems and possible causal factors founded on state agency power...