In his article, “Just War, As It Was and Is,” James Turner Johnson makes a compelling argument for a modern return to the just war theory advocated by Thomas Aquinas in the 13th century. Aquinas' theory is relatively simple. Warfare is just when three conditions are met: sovereign authority, just cause, and right intention. Johnson, speaking on behalf of Aquinas in a way, defines sovereign authority as “a person in a position of responsibility for the good of the entire community,” just cause as “defense of the common good,” and right intention as “both the avoidance of wrong intention and the positive aim of securing peace.” A just war, then, is one began by someone in a position to defend the public good against those who threaten it and in such a way not for personal glory but to protect and ensure that public good. While this theory on when war is acceptable is a wide derivation from what modern intellectuals believe, specifically in its lack of wanting to avoid war at all costs, the theory itself does have its merits. Simply because Aquinas and Johnson see no inherent presupposition against warfare does not mean they necessarily advocate it; it simply means that they are not afraid to use it as a tool for the betterment of mankind.
Aquinas' theory, and Johnson's advocacy of it, is a certain departure from the modern views of war. Modern warfare is generally seen as so violent and so immoral that it is commonly only acceptable as a last resort, when the methods of politicians and diplomats have been exhausted and no other peaceful means of ending conflict remain. In this modern context, however, just war theory has its benefits. While recent modern genocides have been largely ignored, just war theory presents a methodology under which other nations could step in and stop them. For example, if the President of the United States decided to act militarily in order to stop the genocide in Darfur, as long as he did it for the benefit of those suffering under the genocide and not for his own personal benefit, then the war is just. Under the modern framework of the steps to warfare, international permission would be required, which would most likely also coincide with a diplomatic and humanitarian effort. To put it simply, just war theory is far more effective in decisively dealing with the evils of the modern world, while the current milieu in dealing with the steps to military action seem slow and tedious, doing as much to protect the tyrants whose people suffer as to push them out of power.
In a perfect world, just war theory would be the ideal methodology for war-making. In this non-perfect world, however, one can see its limitations. Realistically, the lines between good and evil are not clearly defined as Thomas Aquinas saw. The world is far more complicated and intertwined than it was during Aquinas' time. What is evil to some cultures is not necessarily evil to others. Theoretically, a just war brought by one country onto another could lead a third country to bring their own just war onto the first. This is where a presumption against warfare seems appropriate. Assuming that war is undesirable, nations are less likely to become entwined in conflicts that spiral out of control; by making sure they have international approval, nations can avoid messy escalation. In this way, the war is not simply just but also prudent. It makes sense to use warfare in a just situation that nations agree upon.
It seems likely then that just war theory works best when there is a presumption against warfare. The three criteria for the allowance of war seem to be the three criteria that make going to war correct and, in this sense, Johnson is correct in wanting to bring back just war theory to the modern mindset. If just wars were talked about more frequently and more strongly, perhaps some worldly evil would shrink away simply to avoid the possibility of a just war. Unfortunately, there are other criteria that make going to war prudent and sensible. As long as diverse nation-states with the power to quickly destroy each other exist it is absolutely necessary for nations to go beyond fulfilling just war theory when taking up arms. They must make sure that they will not lead an escalation of violence beyond anything conceivable during Aquinas' time. So, while Aquinas' just war theory is an entirely appropriate tool in modern times, it should never be the only one, if for no better reason then because it is not a modern theory, and does not take into account the complexities of the modern world.
 Johnson, James Turner. “Just War, As It Was and Is.” First Things. (January 2005)