Some are now suggesting that President Obama and Secretary Kerry’s real gambit, in response to the Cotton letter, is to make a nonbinding deal, and then ask the U.N. Security Council to bless it. Whether the U.N. approves of any deal has no effect on the agreement’s legality under our domestic law. The Supreme Court made that clear in a series of death-penalty cases, ending in Medellin v. Texas, where the Supreme Court made clear that Texas could refuse to obey an order by the International Court of Justice (whose authority also comes from the U.N. Charter). Of course, the Supreme Court had held earlier that the Supreme Court was not bound by the ICJ either. Neither the Security Council nor the ICJ can alter our Constitution’s separation of powers, including its division of roles in the approval of international agreements.
As a question of international law, the United Nations Security Council could always order the United States, and any nation, to undertake some action. Refusal to obey a command of the Security Council could be considered a breach of the U.N. Charter. It should be noted that Iran is already in breach of U.N. Security Council resolutions ordering it to halt its nuclear-weapons programs. But whether any Security Council action on a sole executive agreement could give it some kind of binding agreement would depend on what the resolution says. Usually, the Security Council does not command action by its members; instead, it requests that they assist. Nations are usually free to choose to decline to assist. When the Security Council approves sanctions or even authorizes hostilities, it does not command nations to sanction or fight — it merely requests that nations choose to help. Nations often do not, and there is no breach of international law.
This points out the real way that Obama could try to end-run around Congress. He does not even need an agreement with Iran. He could just go to the U.N. Security Council and ask it to enact a resolution that would call for the quid pro quos of his ideal agreement. The Security Council could ask nations to drop its sanctions. It could also ask Iran to stop its nuclear program (basically repeating itself). It could call for international inspectors. But this would amount to a desperation move, because it would have no effect on Congress’s ability to impose mandatory sanctions or a future president’s right to violate any agreement with Iran, real or secret.