Radio Free Asia contributor and Vietnamese dissident Truong Duy Nhat is interviewed in Washington at the Radio Free Asia studio on May 31, 2016. (Radio Free Asia/AP)

EXILE IS becoming increasingly hazardous for writers and other activists who challenge authoritarian regimes. First Russia and China, and then lesser powers such as Saudi Arabia, have demonstrated that they can kidnap and even kill their opponents in other countries — and get away with it. If the impunity continues, other dictatorships inevitably will join in. The latest case concerns a distinguished Vietnamese blogger who disappeared in Thailand the day after requesting asylum from the United Nations.

Truong Duy Nhat had been writing regularly for the U.S. government-supported broadcaster Radio Free Asia since his release from prison in 2015. Last month, according to associates, he grew concerned that he was being targeted for arrest by the government of Nguyen Xuan Phuc, possibly because he was believed to have obtained damaging information about the prime minister. He traveled to Bangkok and on Jan. 25 requested asylum at the office of the U.N. High Commissioner for Refugees. The next day, he was seized by several men at a shopping mall. His whereabouts remain unknown, but Radio Free Asia officials and human rights groups believe he may have been abducted by Vietnamese agents and returned to that country.

If the reports are true, the damage won’t be only to Mr. Nhat. Bangkok has been a harbor for dissident journalists from other Southeast Asian nations, including Vietnam. Mr. Nhat’s disappearance suggests that none of them is safe. More broadly, if nations fail to protect refugees and other exiles inside their borders, the world will become increasingly lawless and unstable.

Thailand has already tolerated or abetted abductions by China, including of the Hong Kong bookseller and publisher Gui Minhai. Last week, Thailand said it had no record of Mr. Nhat entering the country and promised to investigate what happened to him. That statement was welcomed by the State Department, which said it would closely monitor the case.

It should. The Trump administration so far has been all too willing to tolerate repression by the Vietnamese regime. Though the Phuc government holds more than 200 political prisoners, the administration last year agreed with it on a new three-year plan for security cooperation and invited it to participate in regional military exercises, and has continued to supply it with military aid.

U.S. military cooperation with Vietnam could help to restrain aggression by China in the South China Sea. But it should not come with no strings attached. Protecting a writer for a U.S. government-backed broadcaster ought to be a minimum requirement.