From Jus ad Bellum to Jus ad Vim: Recalibrating Our Understanding of the Moral Use of Force

In the preface of the 2006 edition of Just and Unjust Wars, Michael Walzer makes an important distinction between, on the one hand, “measures short of war,” such as imposing no-fly zones, pinpoint air/missile strikes, and CIA operations, and on the other, “actual warfare,” typified by a ground invasion or a large-scale bombing campaign. Even if the former are, technically speaking, acts of war according to international law, he proffers that “it is common sense to recognize that they are very different from war.” While they all involve “the use of force,” Walzer distinguishes between the level of force used: the former, being more limited in scope, lack the “unpredictable and often catastrophic consequences” of a “full-scale attack.” Walzer calls the ethical framework governing these measures jus ad vim (the just use of force), and he applies it to state sponsored uses of force against both state and nonstate actors outside a state’s territory that fall short of the quantum and duration associated with traditional warfare. Compared to acts of war, jus ad vim actions present diminished risk to one’s own troops, have a destructive outcome that is more predictable and smaller in scale, severely curtail the risk of civilian casualties, and entail a lower economic and military burden. These factors make jus ad vim actions nominally easier for statesmen to justify compared to conventional warfare, though this does not necessarily mean these actions are morally legitimate or that they do not have potentially nefarious consequences.

Just war scholars, however, often do not differentiate between force and war, but rather talk about bellum justum as if all uses of force implied the same moral challenges. The tendency is therefore to evaluate forces short of war through the lens of jus ad bellum. We question whether this assumption is warranted. In particular, we inquire whether jus ad bellum offers a useful moral framework for assessing the acts of force short of war that increasingly characterize global conflict. Thus, in the first part of the article, we articulate the limitations of jus ad bellum principles in evaluating recent trends in international affairs—such as the rise of nonstate actors and the advancements in precision weapons technology (for example, drones)—that have weakened the sovereignty norm and facilitated small-scale uses of force to combat perceived threats. We argue that the jus ad bellum framework does not offer sufficient leverage for assessing the jus ad vim actions that have become the hallmarks of the Obama administration’s approach to combating terrorism.

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