NGOs (non-governmental organizations) influence governments, although such influence has not been studied systematically. In this work, the elusive concept of influence is examined and a theoretical and analytical schematic is established for studying the influence that environmental NGOs have on the relationships between different levels of government in Mexico. The case of the construction of a confinement area for dangerous residues in Guadalcázar (near the city of San Luis Potosí) is analyzed. In said case, the relationship between the federal, state, and municipal governments in Mexico is seen as being affected by the external pressure of the US government. Moreover, the influence of San Luis Potosí's civil society, through environmental NGOs whose sphere of attention was concentrated in a localized geographic area, exercised a decisive impact on the development of events, culminating in a formal court battle occurring in the light of NAFTA. In this work, we examine the case of Metalclad in the light of an interdisciplinary theoretical framework, and we define the degree to which the environmental NGOs were able to influence intergovernmental relationships between the federal, state, and municipal levels.