Trump is looking  for a way to ignore reality on the nuclear deal:
After a contentious meeting with Secretary of State Rex Tillerson this week, President Donald Trump instructed a group of trusted White House staffers to make the potential case for withholding certification of Iran at the next 90-day review of the nuclear deal. The goal was to give Trump what he felt the State Department had failed to do: the option to declare that Tehran was not in compliance with the contentious agreement.
The State Department couldn’t honestly give Trump the “option” he wanted because there is no proof that Iran isn’t complying with the terms of the agreement. When all other parties to the deal and the IAEA agree that the deal is working as intended, it isn’t credible to assert that Iran isn’t complying without simply making things up. Trump wants his advisers in the White House to do just that. The trouble here isn’t just that Trump is desperate to find a way to renege on the deal, but that he wants to do so when the deal is doing exactly what it is supposed to do. That is what comes from Trump’s Iran obsession and his reflexive hostility to anything associated with Obama.
Trump is reportedly frustrated with Tillerson because the secretary is incapable of working miracles:
At the previous review in April, Trump had asked Tillerson for specific preparations, which included speaking with foreign allies and to make sure they were on board. “Literally Tillerson did none of this,” the source said. “Simply, [Trump] no longer trusts the State Department to do the work he orders them to do, in order to provide him the options he wants to have.”
The two other sources declined to go into specifics about what Tillerson did not do, only stressing that Trump no longer has faith in the secretary, who simply did not carry out an assignment from him.
Tillerson couldn’t “make sure” allies were “on board” with what Trump wanted because they see no reason to deny that Iranian compliance is happening when they all know that it is. Unlike Trump, they judge the deal on its merits, and they see no benefit in sabotaging it. Trump gave Tillerson an absurd, impossible job, and then faults him for failing to deliver on it. The problem, as usual, is that Trump has been convinced for years that the nuclear deal is the “worst” deal ever made, and nothing Tillerson or anyone else says is able to make him give up on that.
Paul Pillar spells out  what could happen if Trump gets his way next time:
If Trump rejects the truth about Iranian compliance, the most favorable possible outcome would be for Iran and the other five non-U.S. powers that negotiated the JCPOA to try to continue the agreement despite U.S. noncompliance. Even that outcome would have significant negative consequences for the United States in the form of lost business in Iran, lost opportunities to build on the JCPOA in addressing other regional problems, and further isolation of the United States and estrangement from its allies. Less favorable outcomes would involve complete breakdown of the JCPOA and an accelerated Iranian nuclear program, with renewed concern about diminishing breakout time until a possible Iranian nuclear weapon, increased uncertainty about the Iranian program in the absence of the enhanced international inspections established under the JCPOA, and heightened danger of U.S. involvement in a new Middle Eastern war.
There appears to be no one inside the administration capable of explaining the benefits of the deal–and the dangers of reneging on it–as well as this. The administration doesn’t have any committed defenders of the agreement prepared to fight for it, and Trump seems determined to ignore what supporters of the deal have to say anyway. Trump hasn’t scrapped the deal yet, but he clearly wants to and it is just a matter of time before he does. That will be detrimental to U.S. security, the security of the region, and the cause of nonproliferation, and Trump and the U.S. will receive full blame from our allies for undermining the agreement.