Reading the paper these days it seems that a particular change in the nature of warfare is receiving increased attention. Clearly, so-called uses of force-short-of-war are on the rise. Both state and non-state actors employ limited force that remains below the threshold that would trigger an armed conflict. Examples are plentiful, most recently including an attempt to jam satellite communications of British transport aircraft leaving RAF Akrotiri while troops were on board.

Finding responses to the challenge posed by force-short-of-war is a timely undertaking as it raises profound moral questions. Can such action be justifiable? What should a legitimate response look like? Finding answers seems anything but straightforward. On the one hand, insofar as it signals greater constraint, the shift away from major state-against-state warfare toward small-scale uses of force may be welcomed as morally advantageous. On the other, because force-short-of-war appears more limited in nature and therefore containable, states may feel encouraged to use such force more frequently and, thus, create a more violent world order.

The traditional starting point for grappling with such questions has been just war thinking, a rich tradition of thought that spans the millennia. Grounded in the tradition’s dual theme of permission and restraint, just war thinkers have argued about how best to regulate the ever-innovating phenomenon of war. Not surprisingly, the spread of force-short-of-war has attracted their interest and, based on the growing importance of so-called “grey zone” action, it is likely to remain at the forefront of debate in the years to come.

To understand why the morality of force-short-of-war seems to differ from large-scale warfare, the debate about the so-called “war on terror” is enlightening. Oftentimes, this “war” has used limited force only, arguably making it look more like policing than actual warfighting. Consequently, although terrorism has traditionally been considered a matter of law enforcement, the new threat of Islamist terrorism made just war theorists reassess that assumption. For some, the blurring of the lines between peace and war, between the law enforcement and war paradigms, was morally troubling.

In response to this challenge from the grey zone, Michael Walzer suggested a new moral framework that he calls jus ad vim (the just use of force-short-of-war). He argues that “we can’t stop with just war theory” and identifies a need to “maneuver between our conception of combat and our conception of police work, between international conflict and domestic crime, between the zones of war and peace.” Importantly, Walzer imagines jus ad vim as a distinct third moral framework that is independent of the traditional frameworks of war and peace. He also imagines jus ad vim to be in no way limited to the “war on terror” only.

Subsequent just war debate has engaged with various forms of force-short-of-war, including drone strikes and cyberattacks. Most recently, Daniel Brunstetter has developed a comprehensive theory of jus ad vim. That said, the intellectual merit of jus ad vim imagined as a distinct third moral framework besides those of war and peace is contested among just war theorists. Helen Frowe, a leading so-called revisionist just war thinker, rejects jus ad vim as “redundant.” Revisionists argue that there is only one set of moral principles that applies all of the time, rather than distinct principles for different moral domains such as war and peace, or force-short-of-war. Not surprisingly, jus ad vim has become a metaphorical battlefield in what I call the “fight for the just war tradition” between traditionalists and revisionists.

In my own research, I am siding with neither traditionalists nor revisionists all of the time. Regarding force-short-of-war, my ambition is to ensure that both military planners and soldiers are provided with a basic level of instruction regarding the ethical challenges posed by such force. Importantly, grounded in a historical approach to just war, I side with revisionists in the sense that I see no need for a distinct third moral framework of jus ad vim. We may want to refer to just uses of force-short-of-war as acts of jus ad vim, but the inherited just war principles can be taken to assess the morality of such acts. In other words, limited force should still be seen as acts of war. What is needed, I argue, is a renegotiation of just war principles vis-à-vis new circumstances, but this can be achieved within the just war framework. Consequently, I break with revisionists on the idea that there is only one set of moral principles.

So how does such a renegotiation look like in practice? In this short piece, I cannot provide a detailed moral argument about jus ad vim. What I can do, though, is point to the morally important questions that decision-makers should consider before authorizing force-short-of-war. I can also hint at insights from the just war tradition that may inform their decisions. I will do this by reflecting on the Obama administration’s pondering over launching limited airstrikes to punish the Assad regime for its use of chemical weapons in 2013.

Readers will recall the heated debate about Barack Obama’s maneuvering on his “red line.” Noting his unwillingness to commit the United States to another war in the Middle East, the president had ruled out military action unless the Assad regime used chemical weapons against its own people. When the regime, in August 2013, launched a major chemical attack, Obama had to decide about how to enforce his red line. In a remarkable back-and-forth, he first delivered a high-profile speech in which he announced plans for punitive airstrikes. The strikes were imagined as a prototypical act of force-short-of-war, “designed to be limited in duration and scope.” Then, in a move that came as a surprise to many, he struck a deal with Assad and its protector Russia to remove Syria’s chemical weapons without the use of force.

Obama’s decision to backtrack from his red line continues to spark debate. Obama himself would later refer to his decision not to strike as his most courageous foreign policy decision exactly because he obtained what he thinks was a good outcome without having to employ force. Obama’s critics, however, would argue that his failure to enforce the self-imposed red line opened the door to other violations of the rules-based international order, such as Russia’s annexation of Crimea in 2014.

There can be no certain answer about whether Obama or his critics are right. What is certain, however, is that the non-violent removal of chemical weapons was incomplete and it failed to deter the Assad regime from launching further attacks in the years to come. In contrast to his predecessor, President Donald Trump would decide to use limited airstrikes to punish the Assad regime.

Would the Obama administration have had the moral authority to enforce the ban on chemical weapons by force? Let us consider the legal argument first. When making his case for punitive strikes, Obama argued that he would not seek UN authorization because the Security Council was “paralyzed.” Moreover, using limited force to punish Assad for his violation of an international norm would have been no case of self-defense. Consequently, as the imagined strikes did not meet the two exceptions of the UN framework’s prohibition on the use of force, punishing Assad would have violated international law.

Having said that, the legal case does not necessarily overlap with the moral case. The “paralyzed” state of the UNSC was indeed troubling. However, the question decision-makers should ask themselves is the following: Should we violate the norm against the use of force to enforce another international norm? Put differently, should we break international law to uphold the rules-based international order? This is a question of profound moral magnitude, and answering it should not be done lightheartedly. I would tend to argue that before considering non-time-sensitive punitive strikes the UNSC should at least be given a chance to deliberate the matter.

Would punishing Assad have been justified in the first place? It is important to note that in international law and most contemporary just war thinking there is a near consensus about self-defense as the only legitimate just cause. How can this be reconciled with Obama’s intention to let Assad pay a “price”? For classical just war thinkers, punishment was the primary just cause for war. It was one of the lessons of the Thirty Years’ War to limit just cause to self-defense. However, the Westphalian principles also created the moral problem that tyrants could act with impunity within their borders. To my mind, there may be limited instances where punishment can be morally justifiable, and punishing Assad may have been one of them.

Finally, the criterion of right intention cautions against illicit motivations and points to the goal of peace. How can decision-makers avoid giving in to negative passions such as hatred against the perpetrators that inevitably arise when watching children being gassed to death? How can punitive strikes contribute to peace? In the Syria case, right intention points back to the authority criterion. Making a case before the UNSC can help avoid rushed action. Moreover, the classical just war conceptualizes peace not as the absence of violence, but as the maintenance of a just order. Relatedly, the type of peace that can be achieved by limited force is a more modest one, a peace Brunstetter describes as “moral truncated victory.” Punishing Assad for his violation of the chemical weapons convention could arguably have been such a peace.

Force-short-of-war raises many moral questions, and jus ad vim has been an attempt to provide answers. In this article, I have intended to report on the state of the debate and to provide an illustration of how thinking about jus ad vim can inform practical decision-making. As the use of force-short-of-war comes in many forms and is evolving quickly, research on jus ad vim is a worthy undertaking indeed.