International Law and the Use of Force: The Jus Ad Bellum

Abstract

Until the twentieth century, no express prohibition against the use of force existed in international law. However, the legitimacy of armed conflict has been an issue of concern since ancient times. Most significantly, the just war doctrine, which was influenced heavily by the writings of Saint Augustine and Saint Thomas Aquinas, characterized armed conflict in terms of right and wrong, morality and immorality. For Augustine, writing in the fifth century A.D., just wars were those that “avenge[d] injuries, when the nation or city against which warlike action is to be directed has neglected either to punish wrongs committed by its own citizens or to restore what has been unjustly taken by it.”1 850 years later, Aquinas refined this standard by suggesting three criteria with which to assess the use of force. To be just, the use of armed force had to be authorized by the sovereign, be for a just cause (i.e., the other side must have committed a wrong), and the belligerent nation or city had to posses the “right intention,” specifically, “the advancement of good, or the avoidance of evil.”2 Writers from the thirteenth to the seventeenth centuries built on the works of Augustine and Aquinas, but continued to address just war in its theological context. By the sixteenth century, however, the classic formulation of the doctrine was under challenge. Typical was Machiavelli’s celebrated pronouncement: “that war is just which is necessary.”3 With the end of the religious wars and advent of the modern nationstate system, marked historically by the 1648 Treaty of Westphalia, the just war doctrine fell into desuetude. Positivism, an approach to norms based on the practices of states (including agreements between them), emerged to replace it. Its foundational principle, sovereignty, rejected external, non-consensual limits on a state’s prerogative to select the tools of international intercourse, including war, that it deemed best suited to achieve its national interests. Although positivism acknowledged that states should exhaust peaceful remedies before resorting to force, once that occurred, the right to use force was essentially unlimited. The sole exceptions were cased bound by bilateral treaties in which the parties had agreed not to settle disputes forcibly. By the turn of the twentieth century, the legal milieu began to change. The Hague Conventions for the Pacific Settlement of International Disputes of