St. Thomas Aquinas is the modern articulator of the idea that governments are required to follow the same moral principles as the rest of us. This is particularly so in the case of a government that claims its source of power is the consent of the governed. St. Thomas More once put it this way: "Some men say the earth is round and some say it is flat. If it is round, can the King's command flatten it; and if it is flat, can Parliament make it round?"
Of course, the answer to those questions is no; and the reason it is no is that kings and parliaments – all governments – just like all living beings, are subject to the laws of nature. One of those laws was articulated by Aquinas and embraced by More and accepted by Thomas Jefferson and taught by many Judeo-Christian scholars, and was eventually engrafted into treaties and into American law. It is the concept of the just war. In American law and culture, for war to be valid, it must not only be lawful, meaning either declared by Congress or defensive; it must also be just.
What is a just war? For a war to be just, generally, a half-dozen principles must be met.
First, since force destroys, and there is a presumption against its use, the presumption must be overcome by first using all peaceful and viable means and alternatives to war; and it must be clear that these alternatives are fruitless before a war can be just.
Second, the cause must be just; that is, the purpose of the war must be to correct a grave, profound, enduring public evil that directly impairs the freedom or safety of those contemplating war.
Third, only a lawfully competent authority may commence the use of violence, as was not the case when President Johnson bombed North Vietnam or President Nixon bombed Cambodia or President Obama bombed Libya. Thus, the internal laws of the nation using military violence must be crafted so that war is the public policy of the nation, not just the temporary personal preference of whoever is running the government.
Fourth, there must be a probability of success, so that men and women are not sent to certain death for a lost cause.
Fifth, the use of force must be proportional to the harm it seeks to eradicate; thus, no more persons may be harmed by the use of military force than are absolutely necessary to achieve the just goals of the war.
Finally, the war must be fought fairly and ended quickly.
