“I don’t think [the North Koreans are] going to want to negotiate giving up all their nuclear weapons,” he added. “But even if they did … I have no idea how we could verify it.”

Perry experienced one of those missed opportunities first-hand. As Bill Clinton’s defense secretary, Perry had seriously considered military strikes to destroy a North Korean nuclear reactor. But in the late 1990s, Clinton asked him to salvage and expand on a fraying 1994 agreement that shut down the North's rudimentary nuclear-weapons program. Perry traveled to Pyongyang, Secretary of State Madeleine Albright met with North Korean leader Kim Jong Il, and a top North Korean military officer sat down with Clinton in the Oval Office. By the fall of 2000 there was buzz about Clinton himself heading to North Korea to begin establishing normal diplomatic relations with America’s longtime foe in exchange for Kim stopping work on nuclear weapons and long-range ballistic missiles. Then came the 2000 presidential election. George W. Bush took a harder line on North Korea and in 2002 discovered that the North Koreans were clandestinely enriching uranium for nuclear weapons. Perry’s initiative fell apart.

The years since have brought a series of nuclear agreements that at times froze the North Korean nuclear program, but over the long term failed to prevent the North from becoming a nuclear-weapons state. The achilles heel of many of these accords was the Kim government’s refusal to disclose all its nuclear activities and permit outside monitors to verify that those activities had ceased.

“In 1999 we had a chance of getting denuclearization. I do not believe we will get that today,” Perry said. Over the last two and a half decades, “we’ve learned we cannot really trust North Korea. … Therefore I’d put a high premium on verification. If we can’t get verification, don’t value too highly the agreement.”

The Trump administration could aim for a North Korean commitment to limit the number of nuclear weapons in its arsenal and not build new ones, but that “would be extremely hard to verify—approaching on impossibility,” he noted. “Suppose they agreed to turn in what they say are their nuclear weapons. We’re not sure how many they have in the first place, so if they turn in 15 or 20 we’re not sure how many might be left. Secondly, suppose they allow monitors into their centrifuges at [the plutonium reactor known as] Yongbyon. We don’t know where they have their other centrifuges and centrifuges are reasonably easy to hide. So they might be churning away making more highly enriched uranium.”

Establishing safeguards against North Korea transferring nuclear components and technology to other states or non-state actors like terrorist groups would be difficult to verify but still worth pursuing in negotiations, Perry said. (North Korea has a history of proliferating missiles and other materials related to weapons of mass destruction.)