The theory can be broken up into two parts. Jus ad bellum designates the moral criteria (legitimate authority, just cause, right intention, last resort) for engaging in a just war, and jus in bello designates the moral criteria (proportionality, avoiding non-combatants, weaponry) for the appropriate behavior during a just war.
With this hyper-concise background in mind, I read this post by Richard Haass, the President of the Council of Foreign Relations. He believes that the just war theory no longer a helpful device because of the make up of modern military conflict. His major claim against the just war theory is that it is too subjective:
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.
Haass, on the point of subjectivity, is correct, as more hawkish theologians have less and loosened criteria for warfare and dovish theologians have more and stricter criteria - there is no universal, agreed upon list of criteria.
To compensate for just war's shortcomings, Haass offers a new theory, the justifiable war theory. First he makes a distinction, wars of necessity and wars of choice - he, unfortunately doesn't define the terms in this post, but it appears this post is a synopsis of his book, War of Necessity, War of Choice. Wars of necessity, by definition are justifiable, but wars of choice, perhaps:
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.
Haass believes that, under the justifiable war doctrine, the second Iraq War was not justifiable. To which he concludes:
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.While I haven't read the book, so my criticism is somewhat unfair, I still find value and worth in the just war theory, despite its ambiguity and shortcomings. All theologians should receive with skepticism any theory that does not stall the inertia towards warfare, and it seems, to me, that the tacit approval of "wars of choice" can also be subjective and abused, like the just war theory. The just war theory, however, was conceived thousands of years ago, and its conceivers could not have imagined our globalized context and our current military systems and weaponry. As such, a modernizing of the theory must happen (past conceptions of nuclear warfare into the age of extremist, terrorist cells), but whether just war theory maintains theological prominence or gets scrapped for a new theory, like the justifiable war theory, remains to be seen.
Update: Theologians have long mourned humanity's tendency towards warfare but found warfare an inevitable part of the human condition. As such, the just war theory was meant, ideally, to provide a framework of justice to this tragic action. The more I think about the justifiable war theory, the more I am troubled about a movement away from justice and towards a justification of warfare. The issue isn't just one of semantics but one of a profound and differing theological approach to warfare.