When May began, Donald Trump was presiding over diplomatic negotiations that could have delivered twin triumphs for his administration. Now, on Memorial Day, he is reaping the wreckage of talks about Iran, and the North Korea process is in limbo. He has badly strained relations with vital US allies and raised the risk of military conflicts in the Middle East and Asia. He has gone from would-be Nobel Prize winner to commander-in-chief of chaos.

National security adviser John Bolton and Secretary of State Mike Pompeo, the new axis of the President’s national security team, deserve some credit for this mess. Both prefer open confrontation with US adversaries to the more nuanced approaches sold to Trump by their predecessors. But in essence the past month has been a decisive demonstration of Trump’s unwillingness — or, more likely, inability — to grapple with the complexities of international affairs. It is a deficit that already is being skilfully exploited by US adversaries, such as China — and it could lead to catastrophe before we get through another 2½ years.

Trump managed to identify a couple of the most serious foreign policy challenges the country faced when he came to office. In their sole meeting, President Barack Obama warned the incoming president that North Korea and its growing nuclear programme could no longer be ignored. It was evident, too, that Obama’s policy for checking Iran’s pursuit of hegemony in the Middle East was failing. Although Tehran was respecting the letter of the 2015 deal limiting its nuclear programme, it had doubled down on aggression in Syria, Yemen and elsewhere, while exploiting a loophole in the pact to pursue the development of long-range missiles.

For a while, it looked like Trump’s blunt and blustering approach might produce results. Fearful that the President would shred the Iran deal, European governments worked hard to come up with a package that would answer Trump’s objections. They promised multilateral action to sanction and deter Iran’s missile production and its meddling in the Middle East, along with a mutual effort to prevent Tehran from resuming large-scale uranium enrichment when current restrictions expire in the mid-2020s.

North Korea’s Kim Jong Un, too, seemed impressed by the pressure Trump managed to focus on his regime, with the help of China. In January, he launched the first diplomatic initiative of his six-plus years in power; by March he was communicating, through South Korea, the proposal of a summit with Trump, along with a vague promise of denuclearisation.

Like virtually all diplomatic openings, the Iran and North Korea talks did not promise decisive victories. Iran’s regional adventurism and the flaws in the nuclear deal could never be solved in a stroke, and experts on North Korea were virtually unanimous in predicting that Kim was unlikely to surrender his nuclear arsenal in the near term. For Trump, the challenge was to extract the most he could from the negotiations and pocket it. The result could have been partial but significant victories on both fronts: a tougher Western front against Iran, along with a reaffirmation of transatlantic co-operation; and, perhaps, a durable freeze on testing or deployment of nukes and long-range missiles by North Korea, amid negotiations on a long-term peace.

Trump instead blew both negotiations up — and the way he did so was telling. He did not plunge into the weeds of the deal with the Europeans and conclude that it would not be strong enough; he never seriously considered it. Senior European officials who lobbied him later said he appeared unfamiliar with its details. His sole focus, it turned out, was to satisfy a campaign promise by repudiating Obama’s principal foreign policy legacy.

Similarly, Trump never seemed to take seriously the reality that Kim was looking not for a quick deal but for a messy multi-stage process, with denuclearisation a distant endpoint. It may be that his regime would have been unwilling to make any concessions worth pursuing, or planned to repeat its past practice of pocketing US economic favours while cheating on its pledges. But Trump never bothered to investigate Kim’s intentions. Instead he impulsively pulled the plug on negotiations when Pyongyang issued a statement he did not like — blindsiding South Korean President Moon Jae In and other US allies.

Trump is now saying the Korea summit may go ahead, even though there is no sign that Kim has changed his position on denuclearisation. Still, the past month has taught all sides a lesson about Trump, if they did not know it already. He is not up to serious negotiation. He cannot be expected to seriously weigh costs and benefits, or make complex trade-offs. He is good at bluster, hype and showy gestures, but little else. In short, he may be the worst presidential dealmaker in modern history.

Jackson Diehl is deputy editorial page editor of The Washington Post. He is an editorial writer specialising in foreign affairs and writes a fortnightly column that appears on Mondays