“North Korea could make Iran look easy,” Ernest J. Moniz, the former Energy Department secretary and nuclear scientist who negotiated many details of the 2015 deal during the Obama administration, said last week.
“This isn’t ‘Trust, but verify,’” he said, using President Ronald Reagan’s phrase from arms control negotiations with the Soviet Union. “It’s ‘Distrust everything and verify, verify, verify.’”
Success with a relatively small force of inspectors in North Korea, according to former weapons inspectors, depends on the full cooperation of its leader, Kim Jong-un, in opening up the vast nuclear enterprise he inherited from his father and grandfather.
Four years ago, the RAND Corporation, which often conducts studies for the Defense Department, estimated that finding and securing the North’s weapons of mass destruction if the nation fell apart — or into any situation in which it might be hostile to inspection — could require up to 273,000 troops. That is far more than the peak of the American occupying force in Iraq.
Like Iran, North Korea would have to begin any denuclearization pact with a comprehensive listing of all its atomic sites, factories and weapons — which American intelligence agencies would immediately compare with their own estimates.
The complexity of the ensuing verification would depend on how extensive a “disarmament” Mr. Trump has in mind — and whether it matches Mr. Kim’s definition.
Everyone agrees that such a program of disarmament would involve the North’s surrendering its nuclear arms. But American intelligence agencies have vastly differing estimates of the size of the arsenal, be it the C.I.A.’s assessment of around 20 nuclear weapons or the Defense Intelligence Agency’s estimate of about 60. That means it is possible that inspectors would never know for sure if they had found everything.
So far, the Trump administration has not said whether North Korea would have to dismantle most or all of its nuclear-fuel production facilities, which are believed to be much more numerous than Iran’s. Nor is it clear whether the North would have to give up its missiles.
But it is the sprawling nuclear complex that poses the largest problem. Inspectors at the I.A.E.A. are not trained to recognize or handle nuclear weapons; they are basically forensic accountants, keeping track of uranium and plutonium flows through factories and of equipment that can be used to produce nuclear fuel.
Ridding the North of its warheads would require military specialists from the Western nuclear states — including “render safe” teams trained to prevent arms from detonating — as well as possible agreements with China or Russia to take the weapons.
Even so, the agency would probably be overwhelmed.
Last year, Tero Varjoranta, the chief inspector of the Vienna-based agency, told nuclear experts that piercing North Korea’s secretive maze would “require novel and modified monitoring and verification approaches to be developed, and probably implemented, at short notice in a very challenging environment.”
The agency now monitors the nuclear activities of 180 nations with a corps of some 300 inspectors, including the 80 assigned to its Iran Task Force.
Western experts put the North’s total number of nuclear sites — including its main atomic center at Yongbyon, its mountain testing complex and a number of clandestine labs and facilities — at 40 to 100, according to the RAND report.
Iran, by contrast, has about a dozen significant sites and never developed nuclear weapons. Nor did Iraq.
That is just one metric to explain why the denuclearization of North Korea has “no good parallels” in disarmament history, said David A. Kay, a nuclear expert who in 2003 and 2004 led the American hunt for unconventional arms in Iraq.
The goal of nuclear dismantlement in North Korea, Dr. Kay said in an interview, “is far more complex than anything the administration is talking about,” because its atomic complex is so large, so advanced — and in some cases, so well hidden.
The North’s atomic industrial site has some 400 buildings — a vast maze largely unknown to the outside world, and one that is much larger than most American weapon laboratories.
At Mount Mantap, a mile-high peak, the North dug a system of deep tunnels for testing its nuclear bombs. Six have been tested so far.
Experts suspect the most recent, in September, was a hydrogen bomb. There is sharp disagreement over whether the gargantuan blast made the site unusable; new evidence suggests the mountain can support new rounds of explosive testing.
The 2014 RAND study, conducted for the United States Army, imagined the North’s collapse and a military rush to secure and remove its weapons of mass destruction. The troop estimates were driven by the need to secure the nuclear sites, though the study also looked at North Korea’s chemical and biological arms, as well as long-range missiles.
In addition, the study took into account a wide range of expected opposition and hostility from North Korea’s military. It put the American force requirements at 73,000 troops in a situation where it was unclear how they would be received, or up to 273,000 if there was active resistance. Currently, the United States keeps about 28,000 troops in South Korea.
Dr. Kay, the head of the American hunt in Iraq for unconventional weapons, estimated that up to 300 inspectors would be needed in the North. But he said that it would be difficult to find enough specialists with the requisite skills and experience.
“Frankly,” he said, “there’s a limit to the talent pool.”
To succeed with a relatively small force of inspectors, Dr. Kay said the North’s denuclearization program would require the nation’s “full cooperation,” to free inspectors from what he called the arduous task of hunting down clandestine sites and outwitting other aspects of “deception and denial.”
In the end, Dr. Kay said, a successful case of atomic verification would probably rely extensively on the North Koreans themselves — as only they understood the real dimensions of their sprawling nuclear complex.
The inspections, he added, would probably come down to monitoring how North Korean experts go about the denuclearization.
“It’s not going to be like Iraq, where we’d find something and destroy it,” Dr. Kay said. “It’s going to be coming in as observers of what the North is going to do, and that will let you keep the numbers small. It will be looking over their shoulders.”
Dr. Kay said that such an outcome was “probably all right if all you want is a Nobel Peace Prize” — as Mr. Trump has welcomed.
But, he added, it would not be enough to reach the demanding standard of obtaining hard proof that a country has gotten rid of all its nuclear arms and the complicated means of producing them.
“I’m not sure that’s what they mean by denuclearization,” he said.