Realpolitik, the pursuit of vital state interests in a dangerous world that constrains state behavior, is at the heart of realist theory. All realists assume either that states engage in such behavior or, at the very least, are highly incentivized to do so by the structure of the international system. Classical realists remind us, however, that Realpolitik presupposes rational thinking, which should not be taken for granted. Some leaders act more rationally than others because they think more rationally than others. Research in cognitive psychology provides a strong foundation for classical realist claims that Realpolitik requires a commitment to objectivity and deliberation, a particular psychology that few leaders exhibit. A case study of Otto von Bismarck's role in German reunification demonstrates that rationality is the exception, rather than the norm. Even though Prussia was under enormous structural constraints that should have incentivized Realpolitik, the man who would become the Iron Chancellor was isolated because of his foreign policy views. Bismarck consistently disagreed with conservative patrons and allies at home, disagreements that can be reduced largely to his higher degree of rationality.