As President Trump prepares for a sitdown with North Korean leader Kim Jong Un, denuclearization is the No. 1 priority – but it is not the only issue that needs to be addressed at a meeting that could become one of the most historic diplomatic achievements in decades.

Retired four-star Gen. Jack Keane acknowledged in an interview with Fox News that discussions over the North's nuclear program are just the beginning, and that there are also other "critical issues" that he believes will eventually be addressed.

"The resolution of the nuclear issue, while it's the most important issue to be sure and it's the one that threatens the region and the U.S. . . . these other issues are of vital interest," he explained.

"I think we'll go in there with our eyes wide open, and understanding that we're dealing with a liar, a cheater, a manipulator . . . " — Gen. Jack Keane

"I think we'll go in there with our eyes wide open, and understanding that we're dealing with a liar, a cheater, a manipulator. But we are also dealing with a potentially historic diplomatic achievement," according to Keane, who serves as Fox News senior strategic analyst.

Convincing Kim Jong Un to give up his nuclear program in an "irreversible" and "verifiable" way, as the White House and others have demanded, "could be the most historic diplomatic achievement since WWII, if it happens," he added.

Fox News spoke with Keane and other experts about some of those topics, and came up with a list of at least 10 different items that the world will likely want to know more about if the veil of secrecy that surrounds almost all aspects of the Hermit Kingdom is finally lifted.

CHEMICAL AND BIOLOGICAL WEAPONS

In December, Fox News reported on North Korea's "other weapons" that keep experts up at night. "They have a large stockpile of chemical weapons, but the one that gets the least attention, and that I worry most about, is their biological weapons program," said Andrew Weber, the former U.S. assistant secretary of defense for nuclear chemical and biological defense programs from 2009-2014. North Korea has denied producing or stockpiling things like smallpox and anthrax, which can deliver lethal results when used in incredibly small amounts (think ounces, or a few pounds at most).

ASSASSINATION OF NORTH KOREAN OFFICIAL OUTSIDE KIM'S BORDERS

The apparent assassination of the estranged half-brother of Kim Jong Un not only reads like something out of a spy novel, it is a specific and recent case that many experts point to when debating whether or not North Korea actually possesses chemical or biological weapons. Kim Jong Nam died as a result of being exposed to VX nerve agent while in Kuala Lumpur's airport in February 2017. A Vietnamese woman and an Indonesian woman have been accused of murdering him, but have argued in court that they believed they were taking part in a reality television prank when they were given a rag and an unidentified liquid, and asked to smear the man's face.

"Kim and his regime have portrayed themselves as innocent, while they intentionally destroyed our son’s life." — Fred Warmbier

The State Department announced new sanctions in March in response to the incident, stating explicitly that "the government of North Korea used the chemical warfare agent VX to assassinate Kim Jong Nam, in the Kuala Lumpur airport.”

CONCENTRATION CAMPS & CRIMES AGAINST ITS OWN PEOPLE

Human rights commissions estimate there are more than 100,000 people in North Korea's concentration/detention camps, and that upwards of 400,000 people have died in those camps as a result of "torture, starvation, disease and execution."

According to Joshua Stanton of the One Free Korea blog, "these reports, in the context of estimates that North Korea has allowed between 600,000 and 2,500,000 of its people to starve to death while its government squandered the nation’s resources on weapons and luxuries for its ruling elite, suggest that North Korea’s oppression and politically targeted starvation of its people collectively constitute the world’s greatest ongoing atrocity, and almost certainly the most catastrophic anywhere on Earth since the end of the Khmer Rouge regime in 1979."

Stanton argues that this is where the discussions with North Korea should begin, and he thinks he knows a way to hold Kim to account as negotiations progress. "I think we should demand that the NK government allow the transparent delivery of food aid to its 'expendable' people as a test of their willingness to accept transparency," Stanton told Fox News.

DEATH OF AMERICAN OTTO WARMBIER

The death of American student Otto Warmbier served as a stark reminder of the brutality that people experience inside the prison camps as recently as last year. In 2017, the previously healthy 22-year-old was unceremoniously released to his family in the United States after being detained for some 17 months.

Warmbier had been sentenced to 15 years hard labor for allegedly stealing a political banner, but was medically evacuated and suffering from severe brain damage. North Korea denies medical evaluations conducted in the U.S. that suggested Warmbier was tortured, but a lawsuit filed by his parents on April 26 suggests that is exactly what happened. The filing alleges the North "tortured [Otto], kept him in detention for a year and a half without allowing him to communicate with his family, and returned him to them in a non-responsive state and brain dead."

"A key issue not yet covered involves not the remains of Americans, but an account from North Korea for Americans last known alive in their hands and not returned." — Mark Sauter

DISAPPEARANCE OF AMERICAN DAVID SNEDDON

David Sneddon's family hasn't seen or heard from him since his disappearance in 2004 while hiking through China. His parents, as well as sources inside Japan and South Korea, believe Sneddon -- a devout Mormon fluent in Korean who would be in his late 30s now -- was kidnapped by North Korean agents to serve as an English tutor, possibly to Kim Jong Un himself.

The Chinese government claims Sneddon -- an experienced traveler who had served as a missionary in South Korea -- plunged to his death while backpacking through Tiger Leaping Gorge and drowned. But Sneddon's body was never found and his family members -- several of whom retraced his footsteps -- do not believe China's explanation. "There's no evidence of that – zero," said Kathleen Sneddon, noting her son is the "only American missing in China since World II whose body has not been found and whose whereabouts remain unknown."

THREE AMERICANS STILL BEING DETAINED ON QUESTIONABLE CHARGES FROM 2015-2017

Korean-Americans Kim Hak Song, Kim Dong Chul and Tony Kim were detained in three unrelated cases between 2015-2017, and all of them have been accused of a variety of vaguely described "hostile" acts. Gen. Keane says "if Kim Jong Un is serious, he's going to give us the three American hostages relatively soon as a sign of good faith." It appears that could actually be in the works, after President Trump suggested in a tweet on the evening of May 2 that "the past Administration has long been asking for three hostages to be released from a North Korean Labor camp, but to no avail. Stay tuned!" The three were even transferred out of a labor camp last month, according to a report from South Korea’s Yonhap News Agency. (Update - On May 9, 2017 Kim Hak Song, Kim Dong Chul and Tony Kim were released.)

AMERICAN POWs HELD BY NORTH KOREA, AND NEVER RETURNED

The three Korean-Americans detained in North Korea aren't the only American detainees that people are looking for more information about. "There is unquestioned evidence that five specific Americans were held alive by the North Koreans after the Armistice but never returned," says Mark Sauter, co-author of "American Trophies: How U.S. POWs Were Surrendered to North Korea, China, and Russia by Washington’s 'Cynical Attitude.'"

Sauter says President Trump should demand information about the group, known as the Ashley Five, and others like them -- including the South Korean soldiers captured around the same time, believed to number in the hundreds. And that's just scratching the surface, he says. "A key issue not yet covered involves not the remains of Americans, but an account from North Korea for Americans last known alive in their hands and not returned," Sauter said.

JAPANESE ABDUCTEES

Ahead of his meeting with Kim Jong Un at the end of April, South Korean President Moon Jae-in stated his intention to discuss the issue of the Japanese citizens abducted by North Korea decades ago. Japan has suggested as many as 17 of its citizens were snatched back in the ’70s-’80s. Much like the claims from David Sneddon's family, it is believed these citizens were abducted as part of a plan to help teach Japanese customs to North Korean spies. Back in 2002, five of those people were actually allowed to visit Japan. Unsurprisingly, they never returned to the Hermit Kingdom.

BALLISTIC MISSILES & OTHER HEAVY FIREPOWER

In November 2017, shortly before Kim Jong Un kicked off his whirlwind of unexpected diplomacy, North Korea tested a ballistic missile with the capability of hitting any city within the United States. Kim has promised that the testing of nuclear weapons and ballistic missiles is currently on hold, but that doesn't mean the Hermit Kingdom’s arsenal has disappeared. Joshua Stanton points out that it's not just missiles the U.S. and its allies should be worried about. "We should also raise the issue of NK artillery aimed at Seoul," Stanton says.

DECADES OF ANTI-U.S. SENTIMENT

Kim Jong Un may be suggesting that he is coming around to the idea of working with the United States, but it only comes after decades of heavily anti-U.S. rhetoric straight from not only his mouth, but that of his father, and his father's father before that.

In a piece for the History Channel website, one long-time North Korea watcher points out that "for the Kim family, there has never been any doubt who the boogeyman must be. It’s the United States," says Blaine Harden, the author of several books on the Hermit Kingdom and a longtime Washington Post correspondent.

Harden says these sentiments were perhaps best explained by the man known as the Great Leader, who Harden calls the country's "founding despot," Kim Il Sung. In remarks to American journalists in 1972, Harden says the eldest Kim blamed the "strong anti-U.S. sentiments" among his citizens on the fact that "they suffered great damage at the hands of the U.S. imperialists during the war... Since the situation is tense, we cannot but continue stepping up preparations for war. We make no secret of this. Who can guarantee that the U.S. imperialists will not attack this country again? What is most important in our preparations is to educate all the people to hate U.S. imperialism.”

COULD KIM BE PUNISHED?

If and when Kim Jong Un is forced to take responsibility for the issues listed above (among others), there is still a sense that he may escape any kind of punishment because of the gravity of his promise to denuclearize. According to Gen. Keane, "if Kim Jong Un denuclearizes and starts to make other changes, certainly he'll get credit for that and I don't think there will be any attempt to drag him into international court as a war criminal or anything like that."

Fox News' Cristina Corbin contributed to this report.