Drawing on Hans J. Morgenthau, this article argues that a key contributor to the unpeacefulness of the post–Cold War unipolar order was the irrationality of US foreign policy. Post–Cold War US foreign policy was irrational in that it failed to base its strategy on the prudent evaluation of the empirical facts in the social and political context in which it was formulated. Instead, it reinterpreted reality in terms of a simplistic picture of the world as accepted by US policymakers a priori, and sought the use of military force as the sole national security strategy to impose the inviolability of the ideals entailed in this picture. This turned post–Cold War US foreign policy into a self-contradictory endeavour as far as the results were concerned: not only did it confuse desirable for essential interests in standardising the enemy – whether Milosevic, Saddam or Qaddafi – to fit the a priori categorisation, but it also opened a gap between the desirable and the possible. For one thing such an irrational post–Cold War US foreign policy failed to accommodate or annul was the empirical reality of conflicting interests in the social and political contexts upon which it sought to impose its a priori picture. This resulted in consequences that were untenable from the standpoint of US objectives and international peace and security, contributing, overall, to the unpeacefulness of the post–Cold War unipolar order.