Why Nations (Including the U.S. and Iran) Comply With Their Agreements
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.