IN AN act of political vanity and geopolitical folly, President Trump has made one of the most serious national security challenges facing the United States, that of Iran, considerably worse. His announcement Friday that he would report to Congress that the Islamic republic is not meeting the terms of the 2015 multinational accord limiting its nuclear program flouted the reports of international inspectors, the unanimous counsel of his national security team and the appeals of key U.S. allies. His threat to terminate the agreement if Congress and America's allies do not meet his demands for revisions could easily lead to Iran's resumption of a race toward nuclear weapons — a dangerous course that the deal brokered by the Obama administration succeeded in arresting.

The nuclear accord is flawed, including sunset provisions that remove restrictions on Iranian nuclear activities beginning eight years from now. But Mr. Trump's hyperbolic claim that the deal is "one of the worst and most one-sided transactions the United States has ever entered into" is belied by the fact that the regime has gone from being less than a year away from being able to produce a nuclear device, according to U.S. intelligence estimates, to having a fraction of the necessary materiel, and that under close international monitoring. The president's contention that Iran is guilty of "multiple violations of the agreement" is belied by eight reports of the International Atomic Energy Agency as well as statements by his secretaries of state and defense and the chairman of the Joint Chiefs of Staff.

Mr. Trump's advisers persuaded him not to withdraw from the accord directly, but instead to send the matter to Congress, which must vote within 60 days on whether to restore U.S. sanctions on Iran. But the president set a trap by vowing to terminate the deal if Congress did not impose new requirements on Iran, including an end to the sunset provisions. Any U.S. attempt to unilaterally revise the accord is doomed; it will be rejected not just by Iran but also by the other parties to the deal, including Russia, China and the European Union.

Sen. Bob Corker (R-Tenn.), the chairman of the Foreign Relations Committee, says he will introduce amendments meant to satisfy Mr. Trump; he told us that "we have no intention of passing a piece of legislation that violates" the accord. But Democrats, including some who were critical of the original Iran deal, are understandably dubious. Sen. Benjamin L. Cardin (Md.), the senior Democrat on the committee, vowed "not [to] buy into the false premise that it is Congress's role to legislate solutions to problems of [Mr. Trump's] own making."

Mr. Trump promised additional action to address the non-nuclear threats posed by Iran, including its interventions in Iraq and Syria. The administration is justified in imposing sanctions on Iran's Revolutionary Guard Corps. But it appears to have no clear plan to address Iran's military entrenchment in Syria, which is threatening to touch off a new conflict with Israel. Rather than tackling those urgent challenges, Mr. Trump prefers to renounce the legacy of his political nemesis Barack Obama and thereby reopen the one front where Tehran is currently contained. North Korea will take note: The United States cannot be trusted to stick to any nuclear accord.