Nearly a year has passed since the death of Liu Xiaobo, the Chinese dissident who received the Nobel Peace Prize in 2010. Yet his widow, Liu Xia, a poet and artist who was never politically active, is still held under house arrest by a Chinese state determined to suppress Mr. Liu’s legacy. In a rare communication with the outside world, Mrs. Liu recently confided to a friend that she has nearly given up hope of being freed: “There is nothing I fear now,” she said. “If I can’t leave, I’ll just die at home. . . . Using death to defy could not be any simpler for me.”

Her hopelessness is devastating for me. Mrs. Liu hired me in mid-2010 to be her husband’s pro bono lawyer and help get him out of prison. In our last conversation, days before her husband’s Nobel was announced, we discussed what might happen to her and whether she should consider leaving the country. “My place is in China with my husband,” she said—fully understanding what that might mean for her. Shortly after the Nobel announcement, she was placed under house arrest, where she has remained, held virtually incommunicado, ever since.

A United Nations working group on arbitrary detention found years ago that Mrs. Liu’s detention violates international law. She has had a heart attack and reportedly suffers from depression. Unless the international community takes urgent and decisive action, she, like her husband, will soon die in isolation.

Since her husband’s death, Chinese authorities have muted criticism by telling concerned governments Mrs. Liu would be released soon—after last October’s Communist Party Congress, then at the beginning of 2018, after the recent National People’s Congress. If the world fails to act in the face of such brazen defiance of all standards of human decency, it gives China a license to act with impunity.

President Trump has repeatedly demonstrated he wants to press China on some of the thorniest challenges in the bilateral relationship, such as applying tariffs on $50 billion of Chinese imports and restricting Beijing from accessing sensitive U.S. technology. If the U.S. can take an aggressive stand in defense of its interests, it should be equally willing to act on behalf of its values by demanding freedom for Mrs. Liu.

Toward that end, Mr. Trump should impose sanctions on Chen Wenqing, China’s minister of state security, and his bureau heads responsible for imposing this Kafkaesque nightmare on Mrs. Liu. He should privately tell President Xi Jinping that unless Mrs. Liu is freed quickly, he will sign the bill advanced by Sen. Ted Cruz to rename the street in front of the Chinese Embassy in Washington for Liu Xiaobo—a symbolic act that Beijing would experience as a humiliation. Mr. Trump has already declined to follow the disappointing example of President Obama, who threatened to veto the bill. Mr. Trump should join with the leaders of France, Germany and the U.K. to demand—privately, and, if that fails, publicly—that China free Mrs. Liu.

The world’s failure to help its most famous political prisoner—an innocent widow who has been relentlessly punished merely for whom she loved and married—is a stain on our conscience.

Mr. Genser, a Washington lawyer, serves as pro bono counsel to Liu Xia.

Appeared in the June 15, 2018, print edition.