At the beginning of last week, Seoul launched a new round of China diplomacy to encourage Beijing to play a “constructive role” in denuclearizing North Korea. Optimally this marks a new era of cooperation between Seoul and Beijing, but it could also be part of a process leading to China’s colonization of the North.

The move follows President Park Geun-hye’s controversial trip to the Chinese capital in early September to participate in the military parade marking the 70th anniversary of the end of World War II. Washington, worried that South Korea was making itself a Chinese satellite, had tried to discourage her from attending the event.

Short of extraordinary measures, there is little the U.S. can do about Park’s increasingly visible tilt to China. Her mission is to seek the peaceful reunification of the two Koreas—her so-called Dresden Initiative announced in March of last year—and she has moved to enlist the Chinese by charming them into cooperation, seeking to engage them at every opportunity.

Park’s efforts to woo China look like they are paying off. Seoul, not Pyongyang, is Beijing’s friend these days on the Korean Peninsula. It is telling that Xi Jinping traveled to Seoul in July 2014, the first time a leader of the People’s Republic visited the South before going to the North. And in fact, he has yet to visit Pyongyang, the capital of his country’s only formal military ally, in his position as China’s president.

Despite all the smiles, the fear in Seoul is that China will frustrate Park’s vision of a unified Korean nation by sending its army south and either leaving behind a puppet regime or even colonizing North Korea.

The Koreans know what it is like to be colonized. The Japanese first invaded and then obliterated their nation, making it a protectorate in 1905 and formally annexing it five years later. The Allies ejected Japan from Korea but temporarily divided the peninsula at the end of World War II. The division into a Soviet-dominated state, now the Democratic People’s Republic of Korea, and an American-supported one, the Republic of Korea, hardened into the two-state system that exists today.

Every South Korean leader has promoted reunification, but beginning in the early 1990s the commitment became hollow. The population of the South, watching the troubled combination of East and West Germany, recoiled at the cost of absorbing their destitute cousins across the Demilitarized Zone. Park, to her credit, is serious about putting her country back together again.

Two obstacles, however, stand in her way. First, Kim Jong Un, the ruler in Pyongyang, thinks he is the legitimate ruler of all Korea. Yet his position may not count for much: Judging from the especially chaotic and bloody leadership transition from his father, collapse of the DPRK, as Kim calls his miserable state, could occur any day.

The second obstacle is the People’s Republic of China. For centuries, the Chinese have viewed the Koreans as vassals, and they have ruled the northern part of the peninsula, either directly as part of China or through tributary relationships. The border between China and Korea has moved hundreds of miles in both directions over time, and both the Chinese and the Koreans know it can move again.

The Chinese, at least publicly, indicate they are no longer interested in conquering the Korean Peninsula and are willing to let events there take their course. In February 2010, Chun Yung-woo, when he was South Korea’s vice-foreign minister, assured Kathleen Stephens, then Washington’s ambassador there, that China “would be comfortable with a reunified Korea controlled by Seoul and anchored to the United States in a ‘benign alliance.’”

“In the past, it was thought that China feared Korean reunification as it would install a threatening pro-USA regime on its border,” Brock University’s Charles Burton tells The Daily Beast. “Today the new doctrinal thinking is that a reunified Korea would draw Korea away from the United States and into a much closer alliance with China as both would collaborate to develop the substantial economic potential of North Korea.”

Burton, who studies Chinese policy on Korea, believes that, in the event of a failure of the Kimist state, Seoul and Beijing might actually “collaborate closely to install a military government” in the northern part of the peninsula and then financially support it. From South Korea’s point of view, that would work, but only as an interim measure. Eventually, Seoul would want to exercise sovereignty over all Korea.

Many assume Beijing, as the condition for accepting a unified Korea, will demand that American forces be kept south of what is now the Demilitarized Zone. In all probability, the price for China’s acceptance of reunification will be higher: South Korea breaking its treaty alliance with the U.S. and issuing a firm timetable for removal of America’s 28,500 servicemen and women from the peninsula.

But China, being China, will want more. For one thing, as Bruce Bechtol, author of North Korea and Regional Security in the Kim Jong-un Era, points out, Beijing will seek to retain its favorable economic interests in North Korea, such as mineral rights, port rights, access to free-trade zones, and the like.

David Maxwell of Georgetown University notes Chinese enterprises have secured 50- and 100-year leases to the North’s natural resources. And leading Pyongyang-watcher Bechtol, in an email to The Daily Beast, said he thinks Beijing will try to get Seoul to agree to a buffer zone on Korean territory along the 866-mile border with China, where Chinese troops are free to roam—and Korean ones are not.

If Beijing gets all it wants, then, as Singapore-based analyst Eric Teo Chu Cheow noted in the Korean Journal of Defense Analysis, “History is thus perhaps coming round one full circle after 320 years, when the Chinese had a pre-eminent position on the Korean Peninsula.”

Even if China gets all this, however, any deal Park makes with Beijing could get undone at a time of crisis. As famed Korea military analyst Robert Collins told The Daily Beast, “The Chinese are prone to pre-emption.”

In a crisis, the increasingly politically powerful generals of the People’s Liberation Army may bully Xi Jinping to get approval to march into North Korea and stay permanently, despite whatever deals he has made with Ms. Park. After all, there is one imperative that Beijing will want to act on before either South Korea or the U.S. does: China will want to immediately take control of the North’s growing arsenal of weapons of mass destruction, especially its nukes and scattered fissile material. And, as Maxwell states, Beijing needs to find and eliminate “evidence of its complicity” in Pyongyang’s nuclear weapons program.

The Chinese have been planning to intervene in North Korea for a long time. Richard Fisher of the International Assessment and Strategy Center points out that the Chinese military has been modernizing along two principle axes, one of them being the route between Beijing and Pyongyang. “Today the Shenyang Military Region next to North Korea has the most modern mechanized army systems and can call on enormous air and missile forces,” Fisher notes. “It is likely that China is far more ready than the United States to wage the next Korean War.”

In the past, Chinese civilians and military officers were unwilling to talk with outsiders about military intervention. That changed, and a few years back military officers began laying out their plans to move into the North in the event of crisis. Glyn Davies, then America’s chief nuclear envoy, in May 2014 publicly said the U.S. and China were in discussions about “all kinds of contingencies” regarding the peninsula. Davies’s unprecedented disclosure came in response to a reporter’s question about claims by Kyodo News that it had obtained a copy of a Chinese contingency plan for the collapse of the Kim regime.

Kim Jong Un’s generals obviously think the Chinese might move on them. The Korean People’s Army last year reportedly transferred 80 tanks to its newly formed 12th Corps, stationed in Ryanggang province close to China, and had plans to send another 80 armored vehicles. The corps was formed to defend against a Chinese invasion, and the movement of tanks is believed to be the first deployment of heavy armor to the area.

And the North Koreans might be right to worry about their sole ally, China. In August, the People’s Liberation Army massed its own heavy vehicles—self-propelled guns—on the North Korean border at Yanji, in Jilin province.

All this means that Ms. Park, in a crisis, may have to send her forces north fast if she wants to avoid having the Chinese take over North Korea—and thereby frustrate her people’s goal, now more than a century old, of one independent Korean state.