The dangers of arrogance and ignorance are nowhere more starkly illustrated than in Donald Trump’s Oval Office, where anguished advisers seek constantly to steer a rogue president away from international disaster. And now the stakes of his stubborn stupidity have gone nuclear.

Having disparaged the Iran nuclear deal for years — without actually understanding its provisions or even the process that produced it — Trump now sounds determined to abrogate that agreement. In his dismal address to the United Nations last Tuesday, he repeated his usual sneering refrain: “The Iran deal was one of the worst and most one-sided transactions the United States has ever entered into.”

Whatever motivates Trump, beyond his swollen egotism and hatred of all things Obama, he seems unable to comprehend the perils such a tantrum will set loose.

Those perils begin with untold damage to the credibility of the United States among the world’s nations, only partially restored by the Obama administration’s diplomacy after the debacle of the Iraq war. Whatever Trump and his press parrots may say about Iran, the facts are not on his side in his attacks on the nuclear deal or Tehran’s adherence to its strictures. And whatever Trump may believe about his capacity to twist the truth in domestic politics, facts matter in the big world.

The Rouhani government, moderate by comparison with its predecessors, has faithfully complied with the dismantling of its uranium enrichment facilities. We know that because the U.S. State Department, led by Trump appointee Rex Tillerson, has certified Iran’s full compliance — not once but twice, as recently as two months ago. (Someone should tell the president.)

For political reasons, and perhaps to placate their obstreperous boss, administration officials like Tillerson qualify that certification. They say that Iran is in “technical compliance” with the deal, as if there is any other kind. But the regime of metrics and inspections that insure the deal works is highly technical. It doesn’t matter whether the Iranians smile or not.

Nor does it matter, as the White House suggests, whether Iran is pursuing other policies disliked by us and our allies. The nuclear deal was negotiated not by the United States alone, but by a coalition of major powers that included the Europeans, Russia, and China. Their cooperation, which brought Iran to the bargaining table, was based on the exclusion of all extraneous issues from the deal. (Someone should explain that to the president, too.)

So for the United States to abrogate the nuclear deal in the absence of any violations by Iran would represent a betrayal of those international partners — whose cooperation on other issues is vital to our interests — as well as Iran. The consequences are potentially catastrophic, not just in the Middle East but in other zones of conflict such as the Korean peninsula.

Consider the prospect of war with Iran, which unlike Saddam’s Iraq possesses a large, competent, and heavily-armed military force. Any such conflict would threaten to escalate into a broad regional conflict with millions of casualties and untold destruction to the world economy and environment.

But even if war doesn’t ensue with Iran, Trump’s abandonment of the nuclear deal will only encourage other states to build nuclear arsenals and distrust any diplomatic effort to limit those weapons. The obvious and immediate case is North Korea, where the world community is desperately seeking to draw Kim Jong-un into negotiations. Yet why should Kim or any other bellicose dictator engage with the United States or its partners again, on any issue, if the Trump administration now punishes Iran for fulfilling its obligations?

The lesson would be all too clear: The Iranian government should never have undertaken the political and military risks inherent in dealing with the United States and its partners, because the word of the U.S. government is worthless. The stain on our reputation will carry costs to us and our children that we cannot begin to imagine.