Since the formulation of the Monroe Doctrine, United States defined Latin America as his 'backyard' in the XIXth and XXth centuries. However, the global and regional transformations in the XXIst century have weakened the Doctrine, and promoted a new idea of the space, named South America. This paper analyzes the role of Brazil as a regional power, which leads the creation of this new space. Furthermore, it analyzes the role of Colombia, a United States traditional ally, facing the Brazilian leadership. [ABSTRACT FROM AUTHOR]