The warning — the most explicit to date since protests began in the territory — was a stark reminder of who ultimately controls the semiautonomous city. The chief spokesman for the Ministry of National Defense made the comments in a briefing on a document outlining China’s defense strategy. He cited protests on Sunday outside the Chinese government’s liaison office in Hong Kong, which demonstrators painted with graffiti.

Analysts said the warning could inflame, rather than calm, the underlying grievances driving the protests in Hong Kong.

Explainer: Soon after China resumed control over Hong Kong in 1997, the People’s Liberation Army established a garrison of 6,000 soldiers there, but China has never ordered them to intervene in the territory’s affairs.

According to a law detailing relations between Hong Kong and the People’s Liberation Army, the military can intervene in “local affairs” only when requested by Hong Kong’s leaders.

Another angle: Organized crime groups, known as triads, have been blamed for an attack on passengers at a train station last weekend that appeared to target antigovernment protesters. Here’s a look at their history.

The F.T.C. is going after Facebook

In its quarterly earnings announcement, the social media company revealed that it was the target of an antitrust investigation by the Federal Trade Commission, underlining the steady stream of actions that regulators are taking to curtail the power of America’s tech giants.