Should the United States attack Iran if we learn it has begun to enrich uranium to the level required for a nuclear bomb? What about attacking North Korea if it appears too close to producing a nuclear warhead small enough to place inside a missile? Or sending troops into Pakistan if the government loses what little control it has over its western regions and terrorists take hold?
No decision is more fateful than the decision of a government to employ military force. Except in the most clear-cut cases, such decisions are also difficult. As a result, just war theory has for centuries provided useful guidance to policymakers, clergy, citizens, and soldiers alike. But just war theory is too subjective and confining for today's real-world threats.
A more useful concept is that of justifiable war.
Just war theory today is a composite that has evolved from ideas developed by various religious figures. In the 5th century, St. Augustine discussed in City of God the circumstances under which killing could be justified and empires legitimately expanded. In the 13th century, St. Thomas Aquinas laid out a more elaborate just war doctrine in his Summa Theologica. He wrote that three conditions were necessary to make a war just: it must be ordered by a competent authority; the cause must be just; and the combatants must have "a right intention, so that they intend the advancement of good, or the avoidance of evil."
Modern just war guidance involves both the decision to go to war (jus ad bellum) and how to fight one (jus in bello). This latter set of criteria focuses on proportionality (how much force is used), targeting (avoiding non-combatants), and means (avoiding certain classes of weapons).
Most of the debate, however, reflects the more basic decision of when to go to war. Building on the writings of both Augustine and Aquinas, there must be a just cause as well as a decision by a competent authority sanctioning the undertaking. War must be a last resort. There must be a good chance of success. And projected benefits must outweigh projected costs. The theory also holds that all the criteria need to be present before a war can be deemed just and hence undertaken.
One problem with just war theory is that it is too subjective. What constitutes a just cause is in the eyes of the beholder, as are the probability of success and any estimate of likely costs and benefits.
Just war theory is also too confining. Is the United Nations Security Council the only competent authority, or was NATO's approval enough to make the Kosovo war just? Waging war only as a last resort means risking the lives of many while other policies are tried and found wanting.
That's why justifiable war is a more useful concept. Justifiable wars undoubtedly include wars of necessity, that is, wars in which the most vital interests of a country are threatened and where there are no promising alternatives to using force. World War Two and the first Iraq war of 1990-1991 following Saddam Hussein's invasion of Kuwait would qualify, as would wars of self-defense
The question is whether wars of choice can also be justifiable. By definition, wars of choice tend to involve less than vital interests and the existence of alternative policies. Vietnam, Kosovo and Bosnia were all wars of choice. So, too, was the second Iraq war begun in 2003.
Are wars of choice ever justifiable? The answer is "yes" when using force is the best available policy option. The argument that the goal is worthy and that war is the best option for pursuing it should be strong enough to garner considerable domestic and international support. More important, the case should be persuasive that using military force will accomplish more good for more people at a lower cost than diplomacy, sanctions, or inaction.
By this standard, the second Iraq war was not justifiable, as the United States could have done more to contain Saddam though strengthening sanctions. There was a decided lack of international support. And even before the war it was argued and could have been known that the likely costs would be great and the accomplishments modest.
But what about the future? The concept of justifiable war is not simply one for history. Iran, North Korea, Pakistan, Afghanistan -all are potential theaters for new or intensified U.S. military action. The question is not whether they would constitute just wars. That is too impractical a standard. The question in the real world is whether they would be justifiable--to Congress, to the American people, to the world. It is a question President Obama will have to answer.
This article appears in full on CFR.org by permission of its original publisher. It was originally available here.