Donald Trump apparently missed the memo regarding his recent phone call with Russian leader Vladimir Putin, despite instructions reportedly written in all caps: "DO NOT CONGRATULATE."

That the U.S. president did in fact laud Putin on his recent re-election — despite Russia's allegedly fixed vote, as well as new U.S. sanctions against the Kremlin — is alarming former diplomats. Not only because of what it says about a president who may have ignored his briefing notes, but because of what it could mean for his unprecedented meeting with North Korea's Kim Jong-un.

Former nuclear negotiators worry that if, as the Washington Post reported, Trump failed to abide by clear orders from his national security advisers on a phone call with Putin, his off-the-cuff approach to diplomacy could impact delicate, detailed discussions with the nuclear-armed leader in the Korean peninsula.

The summit is tentatively set to happen within the next two months, giving Trump precious little time to master the intricacies of nuclear deterrence. Trump and Kim have never met before, though they have traded barbs about which leader controls a bigger "nuclear button."

Prepping for the summit, former U.S.-North Korea negotiators say, is a time for sober study about the North's nuclear facilities and the pace of its intercontinental missile program. Disregarding guidance from his national security advisers could scupper an opportunity for achieving a breakthrough.

The president's personality is a wild card in the possible U.S.-North Korea talks, said Joel Wit, a former State Department official who worked as a nuclear negotiator with the North Koreans.

"I'm assuming there's a lot of nervousness about what President Trump might say once he gets into a room with Kim Jong-un," said Wit. "And I suspect [Kim] has a much better grasp of the situation, certainly on the peninsula and when it comes to his own country's foreign policy and security threats."

'It's his job to understand these issues'

Trump's call to Putin underscored what Gettysburg College presidential scholar Shirley Warshaw sees as the president's "disdain" for the guidance of career bureaucrats. As a presidential candidate, he reportedly did little prep or review of briefing books ahead of his debates against Hillary Clinton, and as president, he has feuded with his intelligence community and has reportedly skipped reading his daily intel briefings.

"If he had sat down and listened to his national security staff on why you shouldn't say this to [Putin], he wouldn't need to have these cards put in front of him in the first place," Warshaw said. "If presidents are well-versed on an issue, they don't need cards. It does not appear President Trump has been well-versed on the issue."

U.S. National Security Adviser H.R. McMaster, widely expected to be fired in the coming months, reportedly did not specifically remind Trump not to fete Putin for the election. Sources told the Post, however, it was in Trump's notecards, along with notes saying he should condemn the alleged poisoning by Moscow of a former spy in the U.K.

Reacting to the Putin call, Republican senator John McCain slammed Trump for legitimizing a "dictator" who won a "sham election," and for not bringing up election meddling for the upcoming midterms or the recent poisoning of a Russian in the U.K.

Warshaw said the fact that Trump would require notes to remind him not to congratulate Putin, following decades in which the U.S. has stood for free and fair elections, is troubling in and of itself.

"This is the president of the United States. It's his job to understand these issues, and it's his job to spend the time with the right people with the right understanding."

Summits are always choreographed ahead of time. If preparation is done in the right way, if he doesn't deviate from the script, it's possible there could be progress made. - Joel Wit, former U.S.-North Korea nuclear negotiator

Trump defended his congratulatory call with Putin on Wednesday, tweeting that "getting along with Russia (and others) is a good thing, not a bad thing."

But ignoring the guidance of experts now could come at a high cost if the summit between Trump and Kim goes awry.

"We're talking about two unusual human beings, both at the top of governments at countries with nuclear weapons, and who plan to talk about nuclear weapons when both have threatened the other with nuclear weapons," said Robert Gallucci, a State Department veteran who led the U.S. delegation that negotiated the 1994 Agreed Framework to freeze North Korea's nuclear program.

Summit talks can take months or years to prepare for. It took Gallucci more than a year of talks "and slogging through the details of a weapons program in North Korea" before he was able to get the Agreed Framework in place nearly a quarter century ago. The North's weapons program is even more robust now than it was in the 1990s.

This opportunity, then, would not be the time for policy on the fly.

'Don't do that' messaging

North Korea experts agree the gravest threat to nuclear war is the risk of escalation and miscalculation that could set an actual conflict into motion. It's in this environment that Trump must now operate, but freelancing diplomatic ideas, or flouting the advice of experts, would be irresponsible, Gallucci said.

"There's got to be... transparency, benefits flowing to both sides, and all this stuff takes time. Is there preparation? Well, yeah! Lots of it," Gallucci said. "The thing is, [Trump's] outrageous behaviour, his not-to-be-expected moves, that's his thing."

Such high-stakes negotiations are mapped out in detail, said Wit.

"Summits are always choreographed ahead of time," he said. "If preparation is done in the right way, if he doesn't deviate from the script, it's possible there could be progress made. He'll probably have to do a lot of studying."

There may well be foreign stakeholders ahead of the planned summit trying to communicate a "don't do that" message to Trump, Wit said. Except Trump has already demonstrated he has no qualms about dismissing advice about what to bring up, for example, in a phone call with Putin.

Wit imagines that an unpredictable Trump, confident in his own deal-making prowess, could offer an impulsive bargaining chip to Kim that would shock U.S. allies in the Korean Peninsula.

"Trump might say we would withdraw all our troops from the Korean Peninsula, that might be his big idea," he said. "I imagine that everyone is sort of telling President Trump, 'Don't do that,' including the South Koreans, who wouldn't want that to happen, either. That's the big unknown here — that there are certain things he might do that he thinks are great, that would be actually disastrous."