Will Hong Kong jail its first political prisoners this Thursday? If it does — and it looks likely — the event will be a watershed in Hong Kong’s modern history and should set off further alarms about China’s intentions for the territory.

Joshua Wong, one of the three young men bracing for this verdict, has been a hero of his hometown’s democracy movement since he was 14 years old. He’s still too young to buy a beer in the United States. The pinnacle of Mr. Wong’s activism came during 2014, when he and a group of students led hundreds of thousands of their neighbors in an extraordinary 79-day demonstration defending Hong Kong’s autonomy from Beijing’s increasing encroachment. The protest, known as the Umbrella Movement, named for its protection against the elements and police pepper spray, was one of the largest in the city’s history.

Three years later, Mr. Wong and his colleagues, including Nathan Law and Alex Chow, are still being punished for their courage. Several are under threat of bankruptcy thanks to onerous legal fees, and others are being stymied in their chosen fields because of their criticism of the Communist Party. But what is by far most troubling is the prospect that Mr. Wong, Mr. Law and Mr. Chow might be thrown in prison this week on charges for which they have already served sentences.

Last year, the three were found guilty of “unlawful assembly” for entering a fenced-off area outside the city’s government headquarters during the Umbrella Movement protests. Mr. Wong and Mr. Law were sentenced to community service; Mr. Chow was given a three-week suspended sentence. The charge — not to mention the verdict — was yet another sign that Beijing had little intention to honor its commitment to free speech and peaceful assembly.