This dissertation analyzes the foreign policy of Luís Inácio Lula da Silva’s government for South America and the decision-making process for foreign affairs of his cabinet. Specifically, two cases will be analyzed in which the Brazilian government decided to negotiate alterations to energy agreements: the nationalization of Bolivian gas (2006), and the demands of Paraguay to modify the Itaipu Treaty (2008-2009). The aim of this work is to contribute to the debate on domestic actors’ participation in the formulation process of Brazilian foreign policy. In both negotiations, the President, the Ministry of Foreign Affairs, and state agents, such as Ministry of Mines and Energy, Petrobras, Eletrobras, Civil House and Ministry of Finance, participated directly in the decision-making process. This process was also influenced by social agents, such as the Industry Federation of Sao Paulo (Fiesp). The Brazilian government preferred profits over the long-term. Consequently, there is a price of leadership, which Brazil is willing to pay to become a regional leader; a position assumed more clearly in this administration. The nationalization of the Bolivian gas and the Paraguayan demands to alter the Itaipu Treaty, besides its economic consequences, had political connotations, because of the domestic troubles Evo Morales and Fernando Lugo were facing when they decided to pressure Brazil. The successful negotiations became a pledge of political stability in both countries and, therefore, to the region.