When Kansas and Colorado have a quarrel over the water in the Arkansas River they don't call out the National Guard in each state and go to war over it. They bring a suit in the Supreme Court of the United States and abide by the decision. There isn't a reason in the world why we cannot do that internationally. President Harry S. Truman, speech, Kansas City (1945) 1
The rules of the game determine whether cooperation is needed and is viable. Some rules are given by nature. These include the direction of the prevailing winds, the fecundity of a fishery, the migratory habits of the fur seal, and the uptake of carbon dioxide by the oceans. Technology is also given, at least at any one time. Other rules, however, can be manipulated by the players. A treaty, for example, changes the rules under which countries exploit a resource (over time, a treaty can also change technology, by creating incentives for innovation and diffusion). The rules that can be manipulated by treaties are obviously of special interest to this book. But there are other human-made rules—rules that are pretty much given at any one time but that evolve over longer periods of time—that are also important. These are the rules of customary law. Custom often creates the need for a treaty. It also constrains what a treaty is able to do. At the same time, an ineffective treaty system creates an incentive for custom to change.
In the case of the fur seal, custom determined that the United States did not have exclusive rights to the seals beyond the three-mile limit (the tribunal that ruled on this matter reached its decision by interpreting the custom). Custom also defined the territories of all the players in this game, including the three-mile limit. It determined that third parties could not be excluded from sealing outside the three-mile limit (the treaty had to get around this, by making entry unprofitable) and that reflagging was legal. Custom said that a treaty could be negotiated, but it also told each country that participation in a treaty was voluntary—thus making it essential that the four-power treaty promote participation. As suggested by the fur seal example, if the rules of custom had been different, a treaty may not have been needed. Today, for example, customary law recognizes the 200-mile Exclusive Economic Zone (EEZ). Had this existed a century earlier, a treaty to protect the fur seal probably would never have been negotiated. It would not have been needed. Recall that Russia's claim to a territorial limit half this size was sufficient to give it total control of the waters near the seal's breeding grounds.