The United States intelligence agencies' assessments of the size of North Korea's nuclear arsenal have a wide gap between high and low estimates. Size matters and not knowing makes it harder for the US to develop a policy for deterrence and defend itself and allies in the region.
The secrecy of North Korea's nuclear program, the underground nature of its test explosions and the location of its uranium-enrichment activity has made it historically difficult to assess its capabilities.
Some US assessments conclude North Korea has produced or can make around 30 to 60 nuclear weapons, said two US officials who weren't authorized to discuss sensitive intelligence matters and demanded anonymity. Such a wide range affects how the US considers addressing the threat. More North Korean bombs could indicate second-strike capacity and then there are questions about how much nuclear firepower the country could mobilize on a moment's notice.
Estimates by civilian experts cloud the picture even further. Most put the arsenal anywhere from a dozen to about 30 weapons.
"The bottom line is that we really don't know how many nuclear weapons they have," said Bruce Bennett, a senior international and defense researcher at RAND specializing in northeast Asian military issues. "Does it make a difference? Absolutely."
"If North Korea only has a small number — one or two or three — they will not brandish them early in a conflict. If they have 30-plus, they are almost certainly going to consider early use of nuclear weapons in a conflict."
Although remote, the danger of a US-North Korean nuclear confrontation has escalated in recent weeks after Pyongyang's first successful tests last month of intercontinental ballistic missiles capable of reaching the United States
President Trump has traded bombastic threats with the isolated, communist government. Last week, Mr. Trump pledged to answer North Korean aggression with "fire and fury." He later tweeted that a military solution was "locked and loaded" after leader Kim Jong Un was said to be considering a provocative launch of missiles into waters near the US Pacific island of Guam.
If a war were to break out now, North Korea could very well be destroyed. But if North Korea succeeds in building nuclear missiles that can reach the continental US, the equation changes. And having more than a few reliable missiles – long-range ones, plus short-range ones that could, for instance, hit South Korea where 28,000 US troops are deployed – enhance North Korea's leverage.
The risk of mass casualties makes any pre-emptive US strikes problematic, as Trump's own chief strategist recognized in an interview this week.
"There's no military solution, forget it," Steve Bannon said. "Until somebody solves the part of the equation that shows me that 10 million people in Seoul don't die in the first 30 minutes from conventional weapons, I don't know what you're talking about, there's no military solution here. They got us." Seoul is South Korea's capital.
Olli Heinonen, a former deputy director-general at the United Nations nuclear agency, said an arsenal of dozens of weapons might suggest North Korea seeks the capacity to retaliate in a nuclear war. A half-dozen weapons would suggest pure deterrence, said Heinonen, who estimates that North Korea now has enough fissile material for up to 40 weapons – about 10 using plutonium and 30 using uranium.
"When you increase the number, it means normally you're going a little bit more offensive, you plan to have a second-strike capability," Mr. Heinonen said. "Very often it's from submarines and we see North Korea also working with those."
While size is important, Kelsey Davenport at the Arms Control Association thinks the more pressing problem is stopping Pyongyang from further advancing its nuclear program.
"North Korea wants to threaten the United States with a nuclear strike, not actually conduct one, so determining the exact size of North Korea's stockpile of nuclear warheads is far less urgent than de-escalating tensions," she said.
Sen. Deb Fischer, the Republican chairman of the Senate Armed Services Committee's panel on strategic forces, said not knowing the size of North Korea's nuclear program can complicate planning and limit options available to the president. But general principles of deterrence can still be applied, Ms. Fischer said.
"Kim Jong Un is probably less likely to launch an intercontinental ballistic missile armed with a nuclear weapon at the United States, and suffer our overwhelming retaliation, if he knows our missile defense will prevent his attack from succeeding," Fischer said, who has called for more funding for homeland missile defense.
A 2015 study by the US-Korea Institute at Johns Hopkins University's School of Advanced International Studies said North Korea could have up to 100 weapons by the end of the decade. That is, if it had 9,000 centrifuges in operation for uranium enrichment and if a light-water reactor, long under construction, finally came online. Under that projection, North Korea would have 58 weapons by 2017, which is comparable to the high end of the intelligence estimates.
Still, most experts think the number is far less.
"It's possible that they have discovered an additional uranium enrichment facility that we haven't known about," said John Schilling, a consultant with the 38 North website on North Korea at Johns Hopkins. If 60 is the high end, he said, then there "has to be an additional uranium enrichment facility to have produced that level."
Assuming the existence of one or more covert centrifuge facilities, North Korea's inventory of plutonium and highly enriched uranium might have provided enough fuel for 20 to 25 nuclear devices by the end of last year, according to Siegfried Hecker, a nuclear scientist and former director of the Los Alamos National Laboratory who visited North Korea's declared centrifuge facility at Nyongbyon in 2010.
"Almost all in government believe there are two centrifuge plants," said David Albright of the Institute for Science and International Security, who views an arsenal of 60 as "unlikely."
This story was reported by the Associated Press.