Needless to say, a strong moral argument can be made for preferring the rule of law to the law of rulers. If those in power can do anything they like, most of us will not like what they do. Some laws are so unjust that when they are imposed on ordinary people, they should be violated. But allowing those in charge of a government to engage in massive violence and killing in defiance of the law is to sanction all lesser abuses as well, since no greater abuse is imaginable. It's understandable that proponents of war would rather ignore or "re-interpret" the law than properly change the law through the legislative process, but it is not morally defensible.
As Attorney General Lord Peter Goldsmith warned British Prime Minister Tony Blair in March 2003, "Aggression is a crime under customary international law which automatically forms part of domestic law," and therefore, "international aggression is a crime recognized by the common law which can be prosecuted in the U.K. courts." U.S. law evolved from English common law, and the U.S. Supreme Court recognizes precedents and traditions based on it. U.S. law in the 1840s was closer to its roots in English common law than is U.S. law today, and statutory law was less developed in general, so it was natural for Congress to take the position that launching an unnecessary war was unconstitutional without needing to be more specific.
In fact, just prior to giving Congress the exclusive power to declare war, the Constitution gives Congress the power to "define and punish Piracies and Felonies committed on the high Seas, and Offenses against the Law of Nations." At least by implication, this would seem to suggest that the United States was itself expected to abide by the "Law of Nations." In the 1840s, no member of Congress would have dared to suggest that the United States was not itself bound by the "Law of Nations." At that point in history, this meant customary international law, under which the launching of an aggressive war had long been considered the most serious offense.
Fortunately, now that we have binding multilateral treaties that explicitly prohibit aggressive war, we no longer have to guess at what the U.S. Constitution says about war. Article VI of the Constitution explicitly says this:
So, if the United States were to make a treaty that banned war, war would be illegal under the supreme law of the land.
The United States has in fact done this, at least twice, in treaties that remain today part of our highest law: the Kellogg-Briand Pact and the United Nations Charter.
WE BANNED ALL WAR IN 1928
In 1928, the United States Senate, that same institution that on a good day can now get three percent of its members to vote against funding war escalations or continuations, voted 85 to 1 to bind the United States to a treaty by which it is still bound and in which we "condemn recourse to war for the solution of international controversies, and renounce it, as an instrument of national policy in [our] relations with" other nations. This is the Kellogg-Briand Pact. It condemns and renounces all war. The U.S. Secretary of State, Frank Kellogg, rejected a French proposal to limit the ban to wars of aggression. He wrote to the French ambassador that if the pact, ". . . were accompanied by definitions of the word 'aggressor' and by expressions and qualifications stipulating when nations would be justified in going to war, its effect would be very greatly weakened and its positive value as a guaranty of peace virtually destroyed." The treaty was signed with its ban on all war included, and was agreed to by dozens of nations. Kellogg was awarded the Nobel Peace Prize in 1929, an award already rendered questionable by its previous bestowal upon both Theodore Roosevelt and Woodrow Wilson.
However, when the U.S. Senate ratified the treaty it added two reservations. First, the United States would not be obliged to enforce the treaty by taking action against those who violated it. Excellent. So far so good. If war is banned, it hardly seems a nation could be required to go to war to enforce the ban. But old ways of thinking die hard, and redundancy is much less painful than bloodshed.
The second reservation, however, was that the treaty must not infringe upon America's right of self-defense. So, there, war maintained a foot in the door. The traditional right to defend yourself when attacked was preserved, and a loophole was created that could be and would be unreasonably expanded.
In addition, in 1945, the United States became a party to the United Nations Charter, which also remains in force today as part of the "supreme law of the land." The United States had been the driving force behind the U.N. Charter's creation. It includes these lines:
"All Members shall settle their international disputes by peaceful means in such a manner that international peace and security, and justice, are not endangered.