This dissertation concerns the decisionmaking processes of foreign policy which led to the interventions of the United States in the Dominican Republic in 1965 and the USSR in Czhecoslovakia in 1968. I analyze how the superpowers` domestic politics influenced their foreign policies, studying the motivations of numerous internal groups and, mainly, how consensus around the military interventions was produced. I explain that those decisions were a result of considerations about the most appropriate tactics for enhancing or, at least, maintaining the relative power of those States. I try to demonstrate, therefore, that divergences between domestic groups over tactic decisions may hinder a State`s efforts on foreign policy, but those domestic groups have a common interest, which is defined in terms of their State`s relative power. With the results of this analysis, we can evaluate the advantage of neoclassical realism, as opposed to the liberal paradigm, in terms of explaining power when it comes to matters of national security policies.