With Iran unconstrained, Saudi Arabia and others in the region may push to obtain a nuclear capacity. The hard-liners in Iran who never liked the nuclear deal will be strengthened in their bid to destabilize the region, leaving relative moderates like President Hassan Rouhani sidelined. In light of America’s abrogation of its commitments, Russia and China’s position in the region will be bolstered at our expense. Israel may now rush into conflict with Iran, betting it can draw in the United States. Under any scenario, America will be less safe; and in the worst case, we could face the choice of going to war or acquiescing to a nuclear-armed Iran.

President Trump, disdainful of any success of his predecessor, has long been determined to destroy this agreement, even though it has served American interests and won the grudging support of many of its original critics. The president spiked the Iran deal out of spite and hubris, with no viable alternative. He asserts that, with increased pressure on Iran, he can negotiate a better deal. But only someone who knows nothing about Iran would presume that it will pay more under duress to get the same thing it was promised three years earlier.

If Mr. Trump thinks he is sending a strong message of resolve to North Korea, he is again mistaken. Instead, he is demonstrating to a far more advanced and unpredictable adversary on the eve of negotiations that the United States cannot be trusted. He has provided proximate proof that any deal the United States makes, even a successful one, may be tossed aside on the whim of this or any other president. Our negotiating partners, including Japan, South Korea and China, will doubt our credibility when we most need to demonstrate collective resolve. Why would Kim Jong-un give up his nuclear and missile capability when the United States has just demonstrated that, once he does so, it might well renege on the bargain?

The president has just made the most foolish and consequential national security decision of his tenure. Exactly what comes next is unclear, but we certainly will face a far worse situation than today. When the mess materializes, Mr. Trump, per standard procedure, will blame everyone else: his political opponents, his predecessor, the Europeans and the Iranians. But there will be only one person responsible: President Trump, our wrecking ball in chief.