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Earlier this week Iraq announced that a three-month-long U.S.-Iraqi intelligence operation led to the capture of five senior leaders of Islamic State (aka ISIS).

The news is the first major announcement about ISIS since the Iraqi government declared in December that the country’s security forces had driven the terrorists from all of the territory they once held. While remnants of ISIS remain, their threat is significantly diminished, as is their potential for a resurgence.

Just a few years ago, ISIS controlled nearly a third of Iraqi territory. What caused the rapid rollback? The answer may be found in a simple math formula.

The 30 Percent Rule

In 2014, U.S. intelligence estimated there were 31,500 men fighting for ISIS. But Fuad Hussein, the chief of staff of the Kurdish President Massoud Barzani, claimed the number of militant fighters was at least 200,000. “I am talking about hundreds of thousands of fighters,” Hussein said, “because they are able to mobilize Arab young men in the territory they have taken.”

Three years later, Army Gen. Raymond Thomas, head of US Special Operations Command, estimated the U.S. and our allies had killed at least 60,000 ISIS fighters.

Who was right in 2014, the Kurdish leader or U.S. intelligence? Surprisingly, both were likely correct. And combined they may explain why the war with Islamic State is near an end.

Intelligence estimated there were a little more than 30,000 fighters, and yet within three years more than twice that number had been killed. This seems to show, as Hussein claimed, that ISIS had as many as seven or eight times as many potential fighters.

Notice that 60,000 killed out of a pool of 200,000 is 30 percent. A couple of years ago, David P. Goldman noted a historically significant pattern related to 30 percent attrition in warfare:

Nations do not fight to the death, but they frequently fight until their pool of prospective fighters has reached a point of practical exhaustion. In most cases, this involves reaching the 30 percent mark where casualties are concerned.

Wars of this character demarcate many turning points in world history. They include the Peloponnesian War, the Thirty Years War, the Napoleonic Wars, the American Civil War and, at least in some respects, the two World Wars of the 20th century. The 30 percent solution appears yet again in Germany’s casualty figures during the Second World War. Germany lost 5,330,000 of 17,718,714 men aged 15-44 years, or again 30 percent of the total.

Let’s call this the “30 percent rule”—a war will reach the point of exhaustion when one side of the conflict has lost 30 percent of its pool of prospective fighters. If the rule reflects a regularly occurring pattern in military history, then it may have profound implications for how Christians approach justice and warfare.

Qualifications and Clarifications

Before considering how this affects just war reasoning, let’s add a few qualifications and clarifications.

First, the rule does not claim that all wars end with one side losing 30 percent of its prospective fighters. Many, if not most, wars end well before this threshold is reached. This rule merely puts an upper limit on how long a war can last—a limit measured not in terms of months or years but in causalities.

Second, as with all such “rules,” there are exceptions. However, in this case, I believe exceptions are rare. The single outlier that comes to mind is when Paraguay fought a war against an alliance formed by Brazil, Argentina, and Uruguay from 1864 to 1870. About 90 percent of Paraguayan men perished in that conflict—three-times more than the rule would have predicted.

Third, the rule is based on the ability of one side in the conflict to surrender. A nation subjected to genocidal aggression, for example, may lose more than 30 percent to casualties because they lack the power to surrender and prevent further slaughter. Similarly, puppet states controlled by a more powerful nation may be forced to continue fighting past the point where they would have given up.

Christian Theory of War

The Christian tradition of just war theory began in the fifth century with Augustine. The church father’s view of justice in warfare can be summed up by his statement, “We do not seek peace in order to be at war, but we go to war that we may have peace. Be peaceful, therefore, in warring, so that you may vanquish those whom you war against, and bring them to the prosperity of peace.” In the 13th century, Thomas Aquinas built on and expanded Augustine’s thought. Later Christian thinkers have added nuance and commentary on the just war tradition, but the main principles we still use today are those derived from Augustine and Aquinas.

The just war thinking of Augustine and Aquinas is rooted in Romans 13:3-4:

For rulers are not a terror to good conduct, but to bad. Do you want to be unafraid of the authority? Do what is good, and you will have its approval. For it is God’s servant for your good. But if you do wrong, be afraid, because it does not carry the sword for no reason. For it is God’s servant, an avenger that brings wrath on the one who does wrong.

Just war theory includes three main areas: jus ad bellum (the moral requirement for going to war), jus in bello (the moral requirements for waging war), and jus post bellum (moral requirements after warfare is concluded). Rather than provide an exhaustive examination of how the 30 percent rule might affect each area, I’ll simply provide a few thoughts to provoke further reflection.

Application to Jus Ad Bellum

There are six criteria that must be satisfied before the decision to enter into war can be considered just. The primary criterion to which the 30 percent rule would apply is “proportionate cause”: the good of going to war must outweigh the destruction and death that will be caused by warfare. In other words, going to war must prevent more evil and suffering than it is expected to cause.

Would we still consider it worth going to war if there is a high probability that, once started, 30 percent of the enemy’s prospective fighters will become casualties? Are we willing to suffer the loss of 30 percent of our own prospective fighters to achieve victory?

Application to Jus in Bello

The second area, jus in bello, establishes the criteria for justly engaging in warfare. Historically, Christian thinkers have proposed two primary criteria for just execution of war, discrimination and proportionality. The primary criterion to which the 30 percent rule would need to be considered is “proportionality.”

The criterion of proportionality in waging warfare is similar to the criterion of proportionate cause in deciding to go to war. As the ethicist Jean Bethke Elshtain once explained, proportionality “requires that the nature of one’s coercive force should be proportional to any injury sustained or planned, and that at whatever minimal force can be used to do the job should be deployed.”

If we believe the war will continue until 30 percent of the prospective fighters are incapacitated, how does that affect our standard for “minimal force”? Is it better to create a high number of casualties quickly to reduce the effectiveness of the current fighting force and to dissuade potential fighters from joining the war? Would it be just to non-lethally injure large numbers of combatants at one time to speed the end of a conflict?

Application to Jus post bellum

If at the end of the war, the state of peace and justice is no greater than before the conflict started, the conflict cannot be considered to have been worth the violence and death. This is why jus post bellum is an essential addition to the Christian just war tradition.

The ways in which the 30 percent rule affect post-war peace are likely to be indirect. One counter-intuitive application may be that an increased number of casualties might lead to longer periods of post-conflict peace. A study published in the International Studies Quarterly found that government mass killing during war reduces opportunities for the opposition to return to military conflict in the future.

Rather than providing justification for increasing casualties, this requires us to be realistic about what may be required if the 30 percent threshold isn’t reached. For example, inflicting fewer causalities may require more post-war occupation and “nation-building.” As the authors of the study say:

We stress that mass killing is a grizzly and morally appalling tactic. But it does appear to keep a country at peace for a longer duration once a conflict ends. If the international community disrupts these effects of mass killing, it may be inadvertently increasing the likelihood that civil war will recur. Thus, if the international community chooses to intervene in conflicts to protect civilians, member states must also be willing to remain in the country over the long term to help the government and opposition groups refrain from returning to war. Unfortunately, few states have demonstrated an appetite for such long-term commitments.

Need for Christian Reflection

As these brief examples illustrate, the 30 percent rule can complicate our just war reasoning. It can set a higher bar for entering into a war yet may also justify swifter and more brutal tactics once the war is engaged. It might even lead us to decide that long-term occupation is necessary if wars do not end with one side exhausted.

As citizens in a democratic republic, Christians in America will sometimes be called to argue the moral reasons for going to or avoiding war, consider what justice requires of us when engaging in warfare, and advocate for our ethical obligations to our enemy once warfare has ceased.

For these reasons we should give serious consideration to the 30 percent rule. It may be an ugly and offensive rule, but if it’s a frequent and recurring pattern of human conflict, we can’t afford not to consider how it affects our thinking about just warfare.