Much of the latest discourse about a prospective nuclear agreement with Iran—with commentary on whether future U.S. presidents could renege on an agreement, on whether an agreement would be binding or non-binding, and so forth—reflects misconceptions on why nations observe international agreements to which they are party, and misconceptions even of the very nature of international agreements.
It may help to understand this by contrasting observance of international agreements with observance of domestic laws. I may believe a particular law enacted by the jurisdiction in which I live to be bad, and I may see no way in which compliance with this particular law, by myself or even by others, protects or advances my interests—except that I nevertheless am apt to comply because otherwise the sovereign authority involved, in the form of police, prosecutors, and courts, may punish me. At the international level there is no sovereign authority to exact such punishment. What we call international law is basically a set of understandings about international conduct that have come to be seen in general as in the interests of the society of nations. What may motivate a state to comply with international law is not enforcement by some sovereign authority but rather the prospect of reciprocal action by other states—action that will hurt the interests of the first state either because the tenet of international law at stake breaks down or because other states make other harmful responses. In other words, compliance comes from self-interest, defined more broadly than merely avoiding the sort of punishment I would receive for violating a domestic law.
An international agreement reached by two or some small number of states is a codification of some understanding about each state's behavior that is in those states' mutual interest to observe. Such understandings do not need to be codified, and many never are. The international system operates more smoothly and peacefully than it otherwise would because states observe countless unwritten understandings about behavior that would be of interest or concern to other states—regarding, for example, where military forces will be deployed. Codification nevertheless offers important advantages (regardless of what particular form—treaty, political agreement, etc.—the codification takes). It provides greater clarity and precision than is possible with unwritten understandings, and this is especially useful when getting into matters more technical than, say, an understanding that the naval forces of country X always will stay north of latitude Y. Codification makes it easier for states not just to observe current standards of behavior but to move to new patterns of behavior; without a written agreement, such a transition is likely to run aground on concerns about who moves first and uncertainty about exactly where the new limits of acceptable behavior will be drawn. A written agreement also sends clearer and stronger messages to domestic constituencies and provides a framework for any complementary changes in domestic law.
Just as with unwritten understandings, compliance with a written agreement ultimately depends on each of the states involved seeing compliance to be in their own interests. The chief interests concerned in an agreement by two or a handful of states are apt to be more parochial than the interests applicable to the whole community of nations that are involved in most international law. The dependence on continued consistency with self-interest is reflected in the very common use in such agreements of sunset clauses—a recognition that how states perceive some of their interests and the best way of serving them might change with time and changing circumstances. A further reflection of the same thing is another very common provision in international agreements: a clause explicitly providing for withdrawal from the agreement, usually with some advance notification required.
Consider one of the issues raised by that ill-conceived senatorial letter to the Iranians that was aimed at screwing up the negotiations by feeding doubts about U.S. credibility. This was the issue of whether a future U.S. president could or would reverse commitments that the current U.S. president made to Iran. Of course a future president could do that, but as others have observed, this sort of reversal of international agreements made by U.S. presidents has been very rare. It is rare not because of fear of punishment by a sovereign authority, and not even because of some warm fuzzy feeling about international law inside the tummies of presidents. It is rare because it has not been in U.S. interests to do such reversals. The interests involved include the interest the United States shares with others in having some predictability and reliability in international diplomacy—an interest somewhat similar to why stare decisis is an important principle in jurisprudence. The interests also include the more specific benefits that the agreement in question brought the United States in the first place. In the case of the Iran nuclear program, it is hard to conceive of any defensible reason that, if Iran had complied for at least a couple of years with an agreement that kept that program tightly restricted, heavily monitored, and entirely peaceful, any U.S. president would say it was in the U.S. interest to destroy that agreement.
As for the reasons Iran would comply with such an agreement, it again is all a matter of self-interest. The talk about differences between binding and non-binding forms of agreement are mostly beside the point. Iranian leaders will sign an agreement and will comply with it if they see the future that it prescribes—of a tightly restricted, heavily monitored, and entirely peaceful nuclear program coupled with relief from economic sanctions and international ostracism—as being more in their interests than the alternative. The fact that they have entered these negotiations and already observed for over a year the restrictions placed on them in the preliminary agreement of November 2013 indicates that they do see their interests that way.
Understanding these principles about international agreements clarifies why it is a fundamental mistake to try to squeeze and punish Iran still more in the hope that it would capitulate on outstanding issues about the nuclear program—and a mistake not only because there is no prospect that such capitulation would ever happen. Even if it were to happen, the result would be an agreement that Iran would have meager incentive to observe for long. The future of such an agreement would be bleak. U.S. interests will be served by an agreement that protects U.S. nonproliferation objectives and that Iran, as well as the United States, wants to uphold.