With the emergence of transnational threats (terrorism, drug trafficking, etc.) and intra-state conflicts since the 90’s, regional organizations started to play a growing role in addressing these issues. Indeed, as growing interdependence revealed the short reach of states’ internal policies, regional missions have been seen as a solution to recover a degree of effectiveness in dealing with these challenges.

However, the European Union is considered a sui generis case for the elaboration of a regional foreign and security policy, emerging from Europe’s history, its deep economic integration and the relative power balance between the states of the region. A common view, in a neorealist or neoliberal perspective, is that an important asymmetry of power in a regional organization would impede such process to take place: small or middle states could ‘bandwagon’ on an hegemonic state’s foreign policy, but the hegemonic state would not consider any other interests than its own. Nevertheless, a constructivist approach would say that states’ interests and identity could evolve through discussion, interaction, socialization, social learning and norm inculcation. In theory, even a hegemonic regional power could be constrained by regional norms and can develop regional interests. Thus, the main question of this paper would be: to what extent asymmetries impede regional organizations to enter a process of regionalization of foreign and security policy? To analyze this issue I will use the case of two regional organizations with important asymmetries of power: the Mercosur with Brazil, and ECOWAS with Nigeria, as regional leaders.