“President Trump is expected to announce his decision on the fate of the Iran nuclear deal in a speech on Friday,” and “is expected to ‘decertify’ the Iran nuclear deal forged by the Obama administration and declare that it no longer serves U.S. national security interests,” according to ABC News.  “That decision not to certify the agreement would give Congress 60 days to re-impose sanctions against Iran that were suspended in 2015 as part of the agreement.”
But the legality of such an action would be highly questionable. The State Department opined in 2015 that the nuclear deal, formally known as the Joint Comprehensive Plan of Action (JCPOA), is not legally binding on the United States , which would make withdrawal from the agreement at any time permissible.
That might have been true if the JCPOA had gone no further than the agreement between the parties. But the agreement was subsequently adopted by the United Nations Security Council, in Resolution 2231 , and, according to Article 25 of the United Nations Charter, the “Members of the United Nations agree to accept and carry out the decisions of the Security Council….” 
Article II, Section II of the U.S. Constitution provides that the president has the power, “by and with the Advice and Consent of the Senate, to make Treaties, provided two thirds of the Senators present concur….”  Article VI of the Constitution says, “This Constitution, and the Laws of the United States which shall be made in Pursuance thereof; and all Treaties made, or which shall be made, under the Authority of the United States, shall be the supreme Law of the Land; and the Judges in every State shall be bound thereby, any Thing in the Constitution or Laws of any State to the Contrary notwithstanding.”
The UN Charter is an international treaty that was duly ratified by the United States Senate , and thus is federal law according to the U.S. Constitution. The United States is, therefore, legally bound by the terms of the JCPOA, until there is a superseding Security Council resolution, the U.S. Constitution is amended to eliminate treaties as binding law, or the United States somehow withdraws from the United Nations.
So withdrawal of the United States from the JCPOA would not only violate international law, an authority that often evokes sneers from those who adhere to a philosophy of might-makes-right, it would also violate federal law. What’s more, the suspicion that Iran is not complying with the agreement is insufficient, by itself, to justify withdrawal. Instead, if the United States believes that Iran is not complying, the correct procedure is to first refer the matter to a “Joint Commission” consisting of the parties to the agreement.  It could thereafter withdraw after going through specific procedures that would make it clear if objections were being made in bad faith. But there is no authority to withdraw from the agreement because the current president doesn’t believe that the agreement should have been entered into in the first place, or that it no longer serves American interests.
There are rational reasons for the United States to comply with international law, even though arguments for doing so often fall on domestic deaf ears. International law arises out of commitments previously made, and if the U.S. doesn’t live up to its commitments it simply can’t be trusted. That is not acceptable, even if the current state of affairs is such that the U.S. is not susceptible to military compulsion. Lawlessness on the part of the U.S. today is poor planning for the inevitable future when it will no longer be the world’s dominant power. Now would be a good time to establish adherence to international law as a precedent. Moreover, what incentive do other nations have to enter into agreements with the United States if subsequent administrations will simply breech agreements they don’t like?
As for those who remain unpersuaded, who believe that it is sensible policy for the U.S. to ignore its commitments and violate international law with the impunity that its military power now affords, perhaps they will be persuaded by the fact that the JCPOA is really a feature of federal law by the provisions of the Constitution. To hold otherwise would be to maintain that we need not be subject to the rule of law.