It takes some brass for President Rouhani of Iran to suggest — as he did in the wake of President Trump’s speech at the United Nations — that if Mr. Trump backs out of the nuclear deal, “no one will trust America again.” The truth is that it’s only by decertifying the Iran deal, redeeming the pledge of his winning presidential campaign, that the president can ensure that confidence in America will be maintained.

This is but one angle to a brilliant, blunt, and broad-gauged speech. The president well-marked the issues of Cuba, North Korea, the Chinese expansionism in the South China Sea, Hezbollah, and Venezuela, and the United Nations itself. We particularly liked, though, his warnings in respect of Iran and his reiteration of his dissatisfaction with the accord struck by Secretary of State Kerry and President Obama.

It’s an important angle for the world body — and Mr. Rouhani went to the heart of the issue, even if he got it backwards. The fact is that Iran got hornswoggled by Messrs. Kerry and Obama into thinking their deal would bind America despite the fact that both houses of the United States Congress made it clear that they were opposed. It’s a tragedy for the United Nations that it got roped into the pact itself.

We don’t want to get ahead of the story. Mr. Trump failed to make clear at the United Nations whether he will actually decline to certify compliance when the matter comes under his pen on October 15 (this is supposed to happen every 90 days). If he does deem Iran’s performance under the deal not up to certification, he could directly address the point that Mr. Rouhani has raised.

The New York Sun has been against the Iran deal from the start. We simply don’t want to sign anything with — to be a contract partner of — the regime in Iran. The regime is illegitimate, and, at its core, anti-Semitic, something that President Obama himself — shrewdly, in our view, drawn out by Jeffrey Goldberg — acknowledged. Far better to work for regime change.

Even if there were some redeeming feature to the agreement, though, it would be worth decertifying it just to mark as unacceptable the attempt it represents to make an end run around the clearly voiced dissatisfaction of the United States Congress. It would be the best signal the Trump administration could possibly send to Iran, to the so-called P-5 (permanent members of the Security Council), and to the rest of the United Nations, Mr. Rouhani notwithstanding.