Washington (CNN) President Donald Trump took a huge risk by holding a summit with North Korean leader Kim Jong Un and hailing it as a stunning history-changing success.

Now, with diplomacy with Pyongyang at an impasse and tensions rising, the payout for that gamble may be coming due.

The world is not yet back to the brinkmanship and "fire and fury" rhetoric between Trump and the isolated state that sparked fears of a slide to war last year.

But if rising angst between the US and North Korea turns into a more permanent fracture of the diplomatic initiative that peaked with June's summit between Trump and Kim in Singapore, fears of a conflict that could kill millions on the Korean Peninsula might return.

Apart from the human and geopolitical stakes, Trump would face an acute, personal political embarrassment if the prestige he has invested in the process turns out to have been for nothing. And Republican leaders, keen to paint him as a statesman with difficult midterm elections looming, could become collateral damage.

But the lack of progress is alarming many seasoned Washington foreign policy leaders, and may be a direct result of the lack of ironclad commitments won by the President in Singapore.

"I think we are back at a very tense moment," David Petraeus, the former Iraq War general and CIA director, said Tuesday on CNN's "New Day."

"I think, frankly, that the declaration that was achieved back when the President and Chairman Kim met was probably vaguer than one would have hoped. Denuclearization doesn't mean the same thing to us as it means to them."

The weeks since the summit have tested Trump's diplomatic credibility.

Since he returned home from Singapore boasting that he had removed the North Korean nuclear threat , Kim has taken few meaningful steps toward denuclearization. North Korea appears to be pressing ahead with the production of missile and nuclear material and has not given an accounting of its weapons programs that would be the basis for any meaningful disarmament process.

Last week, Trump suddenly ordered Secretary of State Mike Pompeo not to make a fourth visit to Pyongyang , throwing the diplomatic situation into yet more turmoil. CNN reported that North Korea had sent the administration a letter warning that diplomacy "may fall apart." The letter, which was first reported by The Washington Post, left the diplomatic process on a precipice and took the President closer to a looming decision point on whether to crank up political and military pressure on Kim or to offer concessions to pick the deadlock.

The latest developments have also put Trump in a tough political position, as critics argue that his naivete in holding a summit with no preconditions, in the absence of a planned out diplomatic process with a ruthless and resourceful foe, has backfired.

"One of the problems is that this administration doesn't seem to want to remember history. This is the fifth time we've been in negotiations about nuclear weapons with North Korea, and the dance has been pretty much the same each time," Maine's independent Sen. Angus King said Tuesday on CNN's "New Day."

New Hampshire's Democratic Sen. Jeanne Shaheen said the diplomatic slump "speaks to the lack of the President's preparation before the meeting."

The criticism has the administration on the defensive.

"This is going to be a tough process, but this is still going in the right direction," US Ambassador to the United Nations Nikki Haley insisted on Tuesday.

Events test upbeat White House message

The shifting politics of the North Korea diplomatic opening are, however, leaving open the question of how long the administration's line -- that it has stopped Pyongyang's nuclear and missile tests and won the return of the remains of US soldiers killed in the Korean War -- is sustainable.

After all, while those concessions from North Korea are welcome, it's obvious that the fundamentals of the threat -- Pyongyang's arsenal of atomic weapons and missiles -- have not fundamentally changed.

As the situation deteriorates, and apparently seeking to insulate himself from political damage, Trump has taken to blaming China for easing up on North Korea as a tactic to increase pressure on the US in an unfolding trade war.

Following the cancellation of Pompeo's visit, Defense Secretary James Mattis on Tuesday declined to commit to extending a suspension of annual military maneuvers between South Korea and the US, a concession Trump made at the Singapore summit that shocked his own administration.

The administration, while noting that it did not lift sanctions on North Korea, appears to be trying to claw back the leverage that Trump ceded by meeting Kim -- a huge expenditure of diplomatic capital since the isolated state has for decades sought equal billing with the United States on the global stage.

The stalled diplomacy leaves Trump with a set of fateful questions.

Is the Singapore summit now exposed as a total bust and a disastrous error by the President? Is the US back on an inevitable path to confrontation with Kim? Will Trump pay a political price for assuring Americans he'd solved the nuclear crisis in a few hours in Singapore? And what are the flashpoints that could pull the two sides towards confrontation?

The problem with assessing what happens next is that the administration's approach on North Korea is almost as inscrutable as the maneuverings coming out of the world's most closed state.

It is impossible to know whether Kim is turning his back on diplomacy or is trying to play hardball with Trump in the knowledge that he has substantial prestige invested in the process.

From the US side, it's unclear what Trump and his team offered the North Koreans in the talks, and whether they are open to a reciprocal process to move toward US recognition of North Korea and a peace treaty to end the Korean War in return for moves on denuclearization.

Without such an approach -- which has in the past been viewed skeptically by officials like national security adviser John Bolton -- meaningful North Korean concessions seem unlikely, and antagonism across the Pacific seems certain to spike.

Keeping a line open to Kim

What is clear is that Trump -- partly out of a desire to avoid a self-made October surprise in the form of a sudden national security crisis on the Korean Peninsula ahead of the midterms -- still has a strong incentive to keep diplomacy alive. By pulling Pompeo's trip, he called Kim's bluff.

But in a tweet on Friday, the President also sought to avoid closing off what he had gushed in June was a warm, personal bond with the North Korean tyrant.

"I would like to send my warmest regards and respect to Chairman Kim. I look forward to seeing him soon!" Trump tweeted.

...Secretary Pompeo looks forward to going to North Korea in the near future, most likely after our Trading relationship with China is resolved. In the meantime I would like to send my warmest regards and respect to Chairman Kim. I look forward to seeing him soon! — Donald J. Trump (@realDonaldTrump) August 24, 2018

As unorthodox as it may be, that relationship between the two leaders could be one wild card with the capacity to stop a slide into open confrontation.

It was the case in June and appears to be now that both men have strong political and diplomatic incentives to keep their channels open -- and that alone means that Trump's decision to hold a summit, despite fierce criticism, may still eventually pay off.

"This was a nontraditional strategy, but sometimes nontraditional strategies work," said Jim Walsh, an expert in North Korea nuclear diplomacy at the Massachusetts Institute of Technology's Security Studies Program.

Walsh argued that it was always unrealistic to think North Korea would throw itself on the mercy of the United States by unilaterally dispensing with its nuclear arsenal.

Perhaps the biggest risk of an escalation lies in the impulsive and prickly natures of Kim and Trump themselves.

Trump is notoriously sensitive to slights and dings to his image or any sense that his achievements are not being respected. Therefore, it will be crucial for Kim, seeking to play on Trump's need for breakthroughs, not to push the President too far or to give him the impression that he is being taken advantage of.