As Trump works to disarm the North Korean nuclear threat, scheduled to meet in Singapore on June 12 with North Korean dictator Kim Jong-un, he may find his biggest challenge lies south of the DMZ border: with President Moon Jae-in.

Not even Pyongyang’s recent announcement that it wants the world to witness its destruction of a nuclear test site should give us any assurances. It has tried this before, announcing the closure of its Yongbyon nuclear facility only to reopen it later.

In the midst of the excitement over the first-ever meeting between a North Korean leader and U.S. president, a downside exists as it is occurring during the very liberal South Korean administration of President Moon Jae-in. To understand his mindset, one need think of pre-World War II British Prime Minister Neville Chamberlain’s efforts to secure peace with Germany’s Adolf Hitler “in our time.”

If South Koreans had elected a conservative as president, Trump would enjoy smoother sailing. But his biggest problem now in pressing Kim to denuclearize is working in tandem with Moon. Should Kim balk over verifiable denuclearization, Moon, not unlike former President Barack Obama, will likely opt for a policy of “strategic patience” — a policy that empowered Pyongyang to get where it is today. We can only shudder over this happening under Moon’s leadership.

In the wake of Moon’s meeting with Pyongyang’s “Pillsbury Dough Boy,” he rides an 83 percent popularity wave—a rate higher than any other South Korean president one year into his term.

Understandably, the vast majority of South Koreans want to avoid war, as did the Brits under Chamberlain. Questionable is Moon’s willingness to play hardball now with Pyongyang to secure a long-term peace. Trump need beware, based on the ideology of Moon’s advisors, the answer may be no.

One whose counsel Moon seeks is Im Jong-seok, his Chief of Staff. Im was a unification activist, in 1989 organizing an unauthorized visit to the North by a 22-year old student, filmed there advocating unification. The incident caused a major uproar in Seoul, eventually earning Im a three-and-a-half year prison sentence for violating national security laws. His reward almost three decades later was greeting Kim’s sister, Kim Yo-jong, when she came to South Korea for the 2018 Olympics.

Im will be a critical player in the North/South rapprochement effort. Both he and Moon blame the current uneasiness between the two countries on South Korea’s prior decade of conservative rule. Such blame is not placed where it obviously belongs: squarely upon the Pyongyang perpetrator.

Of course, Moon and Im ignore the advances made by North Korea under two earlier liberal presidents, Kim Dae-jung and Rho Moo-hyun, embarking upon their ill-fated “Sunshine Policy” (1998-2008) of appeasement towards Pyongyang. The two South Korean presidents embraced it despite demands by many at the time that they should take a stronger stand against the North’s ruthless military provocations against the South. The policy’s abject failure became evident in 2006 when Pyongyang conducted its very first nuclear test.

It should come as no surprise then that Moon is considering three liberal candidates to be a special envoy on North/South reconciliation. In addition to Im, he is considering two holdovers from the days of the country’s Sunshine Policy. They are the South’s current intelligence service chief, Suh Hoon, and its Unification Minister, Cho Myong-gyon.

Suh’s selection causes some concern. He was a player in coordinating the two summit meetings between Kim’s father, Kim Jong-il, and the two aforementioned South Korean presidents, in 2000 and 2007. Concern stems from the fact that the only reason the North agreed to both meetings was the South’s payment of hundreds of millions of dollars. By 2009, recognizing such meetings were of much greater political value to Seoul, Pyongyang demanded $10 billion to meet.

As conservatives also expected, while the summits achieved some tangible results, little was done permanently to ease conflict on the peninsula. (Moon was a player as well in the 2007 summit as President Roh’s chief of staff.)

Moon’s leanings have always swayed left. As a lawyer, he was a legal advisor to a radicalized anti-American Korean teachers’ union. While South Korea was under military rule in the 1980s, many student activists like Moon learned about the North’s “juche” ideology, with which they became sympathetic. This was a concept embracing total self-reliance. (In an act most telling about juche’s failure, however, its primary architect, Hwang Jong Yop, defected to South Korea in 1997—dying thirteen years later a hero in the South and a traitor in the North.)

It cannot be ignored either that Moon rose from obscurity by riding an unfortunate wave of anti-Americanism. In June 2002, two 14-year old South Korean girls were accidentally crushed by US military vehicles conducting training exercises. Faux stories about American soldiers laughing afterward triggered riots.

Fortunately for Moon and then-presidential candidate Roh, this anti-Americanism was still fomenting at election time —and they cashed in on it. They made it clear in the event of a North Korea/U.S. crisis, they would not necessarily side with the latter.

While Moon and company may not feel they are pro-North now, it is hard to change old ways. As such, they do represent a potential stumbling block for Trump should the talks with Kim fail and he looks to impose a harder line policy against Pyongyang. As one of Moon’s fellow activists said, “We’re not North Korea sympathizers … but may have a warmer heart and more patience than others toward peace.”

Dealing with a North Korean leadership to whom a warm heart towards peace is nonexistent, the question still remains whether Moon has the wherewithal to press for more than just a Chamberlainesque peace.

Trump need bear in mind factors are in play by which we may well witness a waxing Moon towards North Korea and a waning one towards the U.S.

Lt. Colonel James G. Zumwalt, USMC (Ret.), is a retired Marine infantry officer who served in the Vietnam war, the U.S. invasion of Panama and the first Gulf war. He is the author of “Bare Feet, Iron Will–Stories from the Other Side of Vietnam’s Battlefields,” “Living the Juche Lie: North Korea’s Kim Dynasty” and “Doomsday: Iran–The Clock is Ticking.” He frequently writes on foreign policy and defense issues.