He had had “misgivings” about the Iran nuclear deal, and had voted against it, Senator Chuck Schumer told WCBS, last Sunday, “but now we ought to see, give it time to work.” This, for Donald Trump, was tweet-bait: “Dem Senator Schumer hated the Iran deal made by President Obama, but now that I am involved, he is OK with it. Tell that to Israel, Chuck!” By “Israel,” Trump presumably meant Prime Minister Benjamin Netanyahu, who greeted with predictable satisfaction the Administration’s unwillingness to certify Iran’s compliance. (“The President has said, correctly, ‘Either fix it or nix it,’ he told CBS’s “Face the Nation.”) And Trump’s remarks likely reinforced his soaring popularity with the Likud’s supporters, who know an uncommon gesture of Presidential affinity when they see one. But if by “Israel” Trump meant Israeli nuclear experts—people who’ve mastered the deal’s details and have led the institutions responsible for the country’s security policy—then Schumer need not bother telling them that the deal should be given time. This is precisely what they are telling him.

Earlier this month, Robin Wright interviewed Uzi Arad, the former head of research at the Mossad and the head of Netanyahu’s own National Security Council from 2009 to 2011. Arad had also launched, in 2000, Israel’s preëminent conference on national security, at the Interdisciplinary Center Herzliya (where we were colleagues from 2002 to 2004). Arad makes it his business to consult with people who are, as he put it, “conversant with the issues”: old intelligence hands (though, he cautioned, nuclear strategy is often not their expertise); civilian-defense officials involved in the procurement and production of relevant weapons systems; arms-control experts, scientists, and engineers who understand the issues of proliferation; and members of the “political class” who have made themselves expert on both the nuclear issue and on foreign leaders’ positions on it. When Wright spoke with him, Arad had been lobbying congressional Republicans to help save the deal; in light of Trump’s announcement, and Netanyahu’s praise for it, I thought I might check back with him.

Arad remains convinced that the agreement served Israel’s interest, because it convincingly stalled the Iranians’ drive to acquire a bomb while providing a diplomatic process within which to address new issues or to refine approaches to old ones. “The J.C.P.O.A.”—the Joint Comprehensive Plan of Action, as the Iran agreement is officially known—“is our only written framework stipulating exactly what constitutes violations, an anchor, a regulatory mechanism for pursuing negotiations or sanctions to manage these threats,” Arad said. And he remains skeptical of Netanyahu’s campaign against it, recalling the conversation he had had with one very senior official, a veteran of the science and defense-policy community, who was in despair about Netanyahu’s call to abrogate the J.C.P.O.A. “I asked him what he thought of the Prime Minister’s policy. ‘Shigaon!’ he told me.” (“Shigaon” is normally translated as “lunacy.”)

Arad is no dove. He supposes that, hovering over the negotiations leading to the J.C.P.O.A., and critical to its limited success, was the Obama Administration’s refusal to renounce the “military option”—a last resort, President Obama said, but consistent with his vow that Iran would never acquire a nuclear weapon on his watch. (Arad told me that he wonders why the Trump Administration has not “referenced the prospect of military force in the same way,” especially because the life span of the J.C.P.O.A. is just ten years, and the clock has been ticking for two.) Indeed, the pragmatism embodied in the J.C.P.O.A. explains why, Arad believes, so many Israeli security professionals favor preserving it. The roster is long—it includes not just Arad but Uzi Eilam, the former director of the Israel Atomic Energy Commission; Isaac Ben-Israel, the chairman of the Israeli Space Agency and the National Council for Research and Development; Ariel Levite, the former deputy director-general of the Israel Atomic Energy Commission; Efraim Halevy, the former head of the Mossad; Amos Yadlin, the former head of the Israel Defense Forces’ Military Intelligence Directorate; Ehud Barak, the former Chief of General Staff (and Prime Minister); Gadi Eizenkot, the Chief of General Staff of the I.D.F.; and many others.

Arad, moreover, ascribes positive motives to what, “in very general ways,” the Trump Administration has done to force the issue of Iranian misconduct. He supposes that most other seasoned Israeli experts sympathize with this effort, and he made clear his respect for the “manifestly experienced” professionals in the Trump Administration’s national-security team, the counterparts of his own interlocutors. And Arad is not indifferent to Iran’s long-range-ballistic-missile program, which was left outside the scope of the agreement yet, in his view, was always “strategically, logically, and functionally” related to Iran’s threatened nuclear capacity. He would like to revisit the closure to inspectors of various military sites that were declared off-limits by Iran subsequent to the J.C.P.O.A. that, he said, “might as well be declared free-to-cheat zones.” And he is alert to Iranian actions that have virtually nothing to do with nuclear issues but are “disquieting”—“terrorism, expansionism, activity in Syria and Yemen”—and are “closer to our borders than to those of any other interested parties.”

Nevertheless, in Trump’s actions (and Netanyahu’s cheerleading), Arad fears the loss of fine distinctions, something that could prove disastrous. “The deal, in its essence, was not symmetrical—each party was not conceding the same currency to the other,” he said. In return for “substantial limitations which have materially slowed down the Iranian nuclear program,” the six powers that negotiated the J.C.P.O.A. afforded sanctions relief. The Iranians bargained for “as much relief for as few concessions as possible on capabilities,” and the six powers drove for “maximum concessions for as graduated relief as they could have to retain some leverage.” With Netanyahu’s blessing, Trump is daring Congress not to upend this framework. And Arad fears that the Republican Congress, drawn to shows of toughness and resentful of Obama’s legacy, may find it harder to resist Trump’s challenge than to hold to a nuanced understanding of the deal’s workings and virtues. The sensible alternative to “fix it or nix it” is keep it and press on other fronts.

“For Congress to impose new sanctions, presumably related to J.C.P.O.A., but really just because the President said the Iranians are not in compliance—the International Atomic Energy Agency has not said that much, Secretary Mattis has not said that much—well, this could be seen both by the allies and the Iranians as an American breach of the deal,” Arad told me. America, not Iran, would find itself isolated. (On Monday, the British Foreign Secretary, Boris Johnson, reiterated that although “Iran is engaged in disruptive behavior in the region,” the deal should “be preserved.”) Ironically, it would be more difficult for Congress to pursue the issues that Arad would like it to pursue—missiles, terror, inspections, sunset clauses—in the absence of the J.C.P.O.A. They require, he said, “tough, separate negotiations,” possibly the threat of specifically targeted new sanctions—“some already under discussion in the Congress”—not a tendentious claim that Iran is not in compliance with the deal. “In order not to play into Iranian hands, and keep the other J.C.P.O.A. powers supportive, American leaders must make a conscientious effort to divorce from the nuclear deal any sanctions entertained in the context of missiles or terrorism,” he said. “Similarly, military pressure in Syria should be divorced from it.”

Arad is concerned, in short, that Netanyahu’s rhetorical campaigns against the J.C.P.O.A.—which Trump has now amplified, and which Republican leaders echo—will undermine the deal, even if its other signatories consider themselves bound by it. “Iran would then be constrained only by membership in the Non-Proliferation Treaty, without the added inspection regime to which it has submitted,” he said. The region would revert to the military brinkmanship that characterized it before the deal, Trump would have to threaten the use of force, and Netanyahu would hold levers that could pull Trump into a preëmptive strike. Speaking to WCBS, Senator Schumer reflected just this apprehension. “The worst things Iran is doing right now are not within the nuclear field but outside” it, he said. (Not coincidentally, perhaps, AIPAC has adopted much the same position. It released a statement emphasizing the need for new, bipartisan coöperation on Iranian misconduct and, while approving Trump’s look at “deficiencies in the JCPOA,” added, with implied favor, “that the United States is not withdrawing from the accord at this time.”)

“Things have to change in Iran, which is why I favor a process of engagement,” Arad concluded. He will always take it for granted that the threat of military force must be credible. Still, it would be reckless, shigaon, to underestimate the institutions of arms control, embodied in such agreements as the J.C.P.O.A.—“the points of contact, the discussible agendas.” He added, “Even when the parties are at odds, zero sum, creating an interface, working to resolve problems, even if done marginally, is a very constructive thing.”