When American doctor Stephen Yoon thinks of North Korea, he does not think of ballistic missile tests or the threat of nuclear war. He remembers instead a 10-year-old girl with cerebral palsy, who suffered from spastic quadriplegia that made her unable to stand or sit. Five years ago, she went to Yoon’s developmental-disability program at Pyongyang Medical University Hospital, where she received treatments from Yoon and his team of local doctors. After almost a year of exercise therapy and some surgeries, she walked out of the hospital on her own.

The event was heralded in North Korean state media as a national victory, but it received no notice in the U.S., where few people even know about the roughly 200 Americans like Yoon who work and live under the rule of Kim Jong Un. Carefully monitored by the regime, they have come and gone for years, doing educational, medical or infrastructural work, and sometimes raising families in a nation that has been officially at war with the U.S. since 1950. Yoon, 45, moved to North Korea 10 years ago. “We were able to convince and convey to the North Korean government that the kids with disabilities have value and they can be part of society,” says Yoon. “I really believe in our presence.”

Heidi Linton, a mother of three from Asheville, N.C., who leads the organization Christian Friends of Korea, has helped to deliver millions in aid to North Korea since 1995 and spends as much as three months a year in the country to support hepatitis and tuberculosis care centers. About 50 other Americans work in North Korea’s Rason Special Economic Zone, near the Russian border, on social entrepreneurship and humanitarian projects. There’s also a predominately American-run school, the Pyongyang University of Science and Technology, that has brought nearly 70 American professors and staff members each semester.

The Americans in North Korea are controversial because they provide services that indirectly help the North Korean regime. But career diplomats say they create a thin but important connection to the Hermit Kingdom. “They are very dedicated aid workers, they care deeply about the North Korean people,” former U.S. ambassador to the U.N. Bill Richardson says of the expatriate community. “We have no diplomatic contact, we have no commercial contact, so some kind of humanitarian contact as a potential bridge to improve the relationship would be helpful.”

But that is set to change. Amid escalating military tensions and after the recent death of Otto Warmbier–the U.S. student who died after he was detained in North Korea–the Trump Administration announced in July that U.S. passports will become invalid for travel in, to or through the country starting on Sept. 1. The official reason for the travel ban is the “mounting risk of arrest and long-term detention of U.S. citizens” by the Kim regime, but the move could signal that Washington is preparing for relations to further deteriorate. North Korea continues to hold three U.S. citizens, including two former staff members of the Pyongyang University of Science and Technology, in its political-detention system.

The news has prompted concern from the small community of Americans who have been working in the country, many of whom are evangelical Christians, a key part of Trump’s base. The very nature of their work is so sensitive and carefully negotiated that they are often reluctant to draw attention to their projects, though the new travel ban has prompted many to break that rule. They say they respect the State Department’s national-security concerns but that the cost of withdrawing aid is severe. “The President has to make a strong stand,” says Franklin Graham, whose global aid organization has done work in North Korea for 20 years. But, he adds, “we’ve got to continue to try to work.”

Many of the Americans who call North Korea home are pushing the Trump Administration for new permission to return. The State Department may allow limited exceptions, but the scope is not yet clear. “It’s an abhorrent moral algebra that has overtaken us, that if moral evil is visited on great numbers, then the plight of individuals–and thus [the] work to relieve the suffering of individuals–somehow doesn’t matter,” says Robert Carlin, a former U.S. official and behind-the-scenes diplomat during the Clinton, Bush and Obama years. “Have we lost our moral compass?”

The U.S.–North Korea relationship has long wavered between delicate and dangerous. Although the Korean War ended in 1953, leaving more than 1 million North Koreans and 36,000 Americans dead, a peace treaty was never signed, and enmity remains. Attempts to restart relations in the decades since have been short-lived, poisoned by distrust. Time and again, nuclear-nonproliferation negotiations have fallen apart or deals have been broken, with tensions spiking, though they have recently receded since North Korea stopped testing missiles in mid-August. As recently as early August, North Korean state media threatened torching the mainland U.S. with “an unimaginable sea of fire.” President Trump, meanwhile, counterthreatened with “fire and fury like the world has never seen.”

The quiet U.S. humanitarian effort in North Korea began as a response to a famine there in the mid-1990s, which killed hundreds of thousands of people. Groups like UNICEF, Mercy Corps and World Vision delivered food aid. When the world’s focus shifted away, Christian charities stuck around and deepened their ties. The Eugene Bell Foundation, which was founded by a Southern Presbyterian minister, began supporting tuberculosis treatment efforts in North Korea around that time, starting the first program for multidrug-resistant TB. “These efforts are really outliers, in part because they are completely going against the grain of the body language that both governments are sending to their people,” says Scott Snyder, a senior fellow for Korea studies at the Council on Foreign Relations.

The new relationships were fragile at the start. A century ago, Christianity thrived in the region, and American evangelist Billy Graham’s late wife Ruth even attended school in Pyongyang. Today North Korea’s constitution allows for freedom of religion, but the country forbids proselytizing. Such subtle distinctions may be lost on an outside world preoccupied with North Korea’s outlaw status: in 2014, the U.N. condemned its leaders for alleged crimes against humanity, including persecution for political crimes with torture, starvation and forced labor. But the U.S. groups are careful to respect the rules, and their focus is service projects.

Christian Friends of Korea grew out of Graham’s visit to North Korea in the early 1990s. Linton, the group’s executive director, traveled to North Korea in August for a routine visit with a team of eight other Americans, three Norwegians and an Australian, all volunteers, to install clean-water systems and continue their hepatitis B treatment program. Linton often visits the two Protestant churches and the one Catholic-heritage church in Pyongyang, but she does not proselytize or preach. Her team of volunteers works alongside Korean officials during every trip. Medical treatment decisions are jointly made, and Americans and Koreans install water-distribution lines together.

Similarly, Franklin Graham–Billy and Ruth’s eldest son and the president of the aid organization Samaritan’s Purse–has sent 30 teams to North Korea over the past 20 years. One team was there in July at the request of the Korea-America Private Exchange Society, an official North Korean organization. It delivered five ambulances, purchased by Samaritan’s Purse, to pediatric hospitals. Both governments allow the presence of humanitarian workers “to the extent that it serves their own purposes,” says ambassador Robert King, the special envoy for North Korean human-rights issues during both of President Obama’s terms. “For the United States, there is a benefit politically. It sort of softens the image of the United States, it provides information about the outside world to North Korea, which is an extremely isolated place.”

Perhaps the most prominent operation in North Korea run by Americans is a university for 600 students started by evangelical Christians. James Kim, an American who immigrated to the U.S. from South Korea, had been involved in food-aid efforts to the country, which briefly got him arrested by authorities in 1998 on spying charges. He dreamed of starting a university that would teach free-market economy and entrepreneurship classes, expose students to Western thought and generate a peace-building movement.

After building a similar school in Yanji, China, in the early 1990s, he worked with North Korean authorities to start a sister school in Pyongyang. It opened in 2010 and was largely funded by evangelicals in the U.S. and South Korea. The Pyongyang University of Science and Technology attracts staff members from the U.S., Europe and other parts of the world, and teaches all classes, including soccer, in English. Graduate students can study abroad in Europe. Many students come from North Korea’s elite families or are chosen by the North Korean Education Ministry. All students live on a closed compound outside the city. The curriculum, says the school’s U.S. director, Norma Nichols, “is designed to open their eyes to other thoughts and to the world.”

Faculty members know their limits. They follow strict rules prohibiting the discussion of politics or religion, and their courses, like international finance management, avoid topics like sociology or culture. The same day the travel ban was posted, on Aug. 2, the school’s North Korean co-president wrote a letter welcoming all foreign professors, their families and administration staffers, and assured the safety of their stay. “We follow the rules,” Nichols says. “We are not going to be detained.”

But recent history suggests the work has risks. North Korea has detained at least 17 Americans in the past decade, and two staffers from the school are among the three U.S. citizens reportedly still held by the North Korean government. Authorities charged each with “hostile acts,” but the reason for their detention remains a mystery–one taught a monthlong accounting course, the other did agricultural development. Neither was a regular employee, school officials say, and they were not arrested on campus but when they were leaving the country.

In other areas, the ruling regime has used detention as a point of international leverage. In March, North Korea blocked Malaysian citizens, including diplomats and their families, from exiting the country after Kim Jong Nam, Kim Jong Un’s half brother, was allegedly assassinated in Kuala Lumpur’s airport. They were released when Malaysian officials eventually agreed to return his body and allow several North Koreans, including suspects in the assassination, to leave Malaysia.

Such behavior has led some in Congress to urge a much more aggressive posture against North Korea. “Tragically, after the horrific treatment of Otto Warmbier and the regime’s demonstrated willingness to use American citizens as negotiating chips, limiting U.S. citizen travel, and ensuring that such travel is reviewed and approved ahead of time, is both sensible and necessary,” Democratic Representative Adam Schiff of California says. Along with Republican Representative Joe Wilson of South Carolina, Schiff introduced legislation in May to stop Americans from traveling to North Korea for five years. Like the State Department ban, which currently runs only one year, there is a narrow carve-out for humanitarian work. A floor vote is expected this year. “It’s a great example of Congress and the White House working in tandem, backing each other up,” Wilson says.

Detention can cost the U.S. government millions of dollars in terms of high-profile diplomatic negotiations and consular efforts, says Snyder of the Council on Foreign Relations, and can prevent progress on critical negotiations. Tourists and religious advocates have generally posed a greater problem than humanitarian workers, who by and large understand the terms of engagement and follow the rules. “I suspect the Trump Administration will tighten those humanitarian contacts, and I don’t think it is very wise,” former ambassador Richardson says.

After the Sept. 1 ban was announced, a representative from the U.S. consulate in Shenyang, China, held a town hall in Yanji, China, near the North Korean border, for U.S. citizens who live in that region. But the meeting provided few details of next steps, which have still not been announced by the Trump Administration. “The safety and security of U.S. citizens overseas is one of our highest priorities,” says Ashley Garrigus, a spokeswoman for the State Department’s Bureau of Consular Affairs. “The travel warning for North Korea is for all U.S. national travelers, regardless of their reason for travel.”

The consequences of violating the ban also remain unclear. The State Department could revoke a violator’s passport, or seek to prosecute those who violate the ban–misuse of a passport is a felony that could result in a long prison sentence. (A Department of Justice spokesperson declined to comment on the potential consequences.) It is also unclear if dual citizens will face repercussions for traveling to North Korea on a non-U.S. passport.

Even if some Americans get exceptions to travel, increased sanctions on North Korea have complicated their work. Yoon says he has been waiting eight months for a license from the U.S. Treasury so he can continue plans for a five-story, $3 million pediatric rehabilitation department at Pyongyang Medical University. It would be the first for the campus, capable of treating up to 200 outpatients and 40 inpatients every day, and funded largely by U.S. and South Korean donations. Christian Friends of Korea has struggled to get the right supplies it needs to make medical cultures and test drug sensitivity. When Linton recently wired money to purchase bicycles for rural medical personnel, the bank froze the funds, and she has spent weeks trying to get them back. “Banks are so afraid of being brought up on violations that they are so inside of the legal red line,” she says. “They don’t want their legal department to be tied up dealing with a small humanitarian organization.”

At the Pyongyang University of Science and Technology, classes are set to begin on Sept. 4 with the remaining non-American faculty. American school administrators plan to leave the compound in late August, after making sure that textbooks, finances and food supplies are in place for the coming semester. But if the Americans are not allowed back in, Nichols fears the school could become a North Korean project. “The chain for all Western influence would be broken at that point,” she says.

Yoon, meanwhile, has been taking his case to Foggy Bottom. “We do not want to see one tragedy turn into a multitude of tragedies,” he wrote in letter to the State Department, using the acronym for the Democratic People’s Republic of Korea. “Humanitarian work in the DPRK requires substantial time and effort in building relationships, negotiating work terms and monitoring the implementation of projects. This requires frequent, if not constant, presence in the DPRK.”

After both growing up in South Korea, Yoon and his American wife Joy met when each moved to Illinois to study biology at Olivet Nazarene University. Yoon became a naturalized U.S. citizen, and when they learned that Christian organizations did work in North Korea, they decided to move there in 2007. He co-founded Ignis Community to support families in North Korea, and his team on the ground now includes 20 Americans, both staff and families. Yoon received special permission from the country’s then leader Kim Jong Il to earn an M.D. and Ph.D. from Pyongyang Medical University, and then permission from Kim Jong Un to develop a spine-rehabilitation program with the Ministry of Public Health. “It took long-term engagement to get permission from DPRK to start our medical programs,” Yoon says. “We have that support, but now our U.S. policy is making us not able to continue to bring life and hope to these children.”

For now, Yoon and his family have decided to keep their apartment at the Polish embassy in North Korea at a reduced rent, but they will wait in their home in China. Even if Yoon gets special permission to return to North Korea to train 28 doctors this fall, he has heard that his family may not be able to join him. “Our cerebral palsy and autism children become a second victim of the political tension,” Yoon says. “I’m hopeful and praying it will happen, but I am not sure with this great tension that the U.S. and DPRK has. They are talking about war.”

This appears in the September 04, 2017 issue of TIME.

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