At a crucial moment, Donald Trump is forcing the world to confront core questions it really shouldn’t have to ask: Can he be trusted? And, more saliently, can America be trusted? His threats to jettison the Iran nuclear deal are undermining America’s credibility as a negotiating partner and weakening America’s ability to lead the free world as it has for 70 years.

In his rush to bulldoze President Obama’s accomplishments, Mr. Trump has withdrawn from the 12-nation Trans-Pacific Partnership trade agreement, leaving China with a freer hand to set trade rules in Asia; abandoned the hard-won 195-nation Paris Agreement to address climate change; and sowed grave doubts about his commitment to NATO, the bedrock alliance that has kept peace in Europe after World War II.

Now, Mr. Trump is threatening to torpedo the 2015 nuclear deal, which imposed strict limits on Iran’s nuclear program in return for a lifting of international sanctions. He has hinted that next month he will not certify that Iran is complying with its commitments, even though the head of America’s Strategic Command just said that it was. The certification is required every 90 days; a failure to provide it could lead to the re-imposition of American sanctions and cause the agreement to unravel.

Perhaps the most immediate consequence of reneging on the Iran deal is that it will make it even harder, if not impossible, for the president to negotiate a peaceful resolution of the crisis over North Korea’s nuclear weapons program. The North’s leader, Kim Jong-un, is erratic and frightening, but why would it help matters to eliminate any confidence Mr. Kim might have in Mr. Trump’s word?