Shujun Wang had long billed himself as a scholar and democracy activist who fled China to build a new life in New York.

But American prosecutors say he was actually a spy for the Chinese Communist Party.

Mr. Wang is on trial this week in Brooklyn federal court on charges that he acted as an agent of a foreign government without registering with the attorney general, as is required by law. He vigorously disputes the charges. If convicted, he could face up to 25 years in prison.

The case is part of a rash of prosecutions taking aim at what the Justice Department calls transnational repression, efforts by the Chinese government to control that country’s vast diaspora through intimidation and harassment.

“The Chinese government will stop at nothing to lie, steal, and cheat its way to wealth and power, to silence those who oppose it, and to project its authoritarian view around the world — and within our own borders,” the F.B.I. director, Christopher A. Wray, said in April 2023 as he announced charges in two new cases.

Wang Wenbin, then a spokesman for China’s foreign ministry, responded that the allegations were a smear and called for the United States to “abandon the Cold War mentality.”

Here’s what to know about the case against Mr. Wang.

Mr. Wang, 75, is an American citizen who settled in New York in the 1990s after a stint as a visiting scholar of East Asian studies at a local university. He was also a leading member of a group in the Flushing area of Queens founded to commemorate the 1989 Tiananmen Square uprising.