A Chinese People's Liberation Army (PLA) soldier in Hong Kong on August 1, 2019. ISAAC LAWRENCE/AFP/Getty Images

The idea of China's People Liberation Army (PLA) on the streets of Hong Kong seemed unthinkable just a few weeks ago. Now it is being considered a possibility.

As Hong Kong's pro-democracy protests grow in frequency and intensity, the commander of the city's PLA garrison said last week that protester violence "should not be tolerated."

Five days of protests: Since then Hong Kong has witnessed another five consecutive days of mass protests, which have shut down stores, disrupted air travel and led to clashes between police and demonstrators. The Hong Kong stock market had its worst day in months on Monday.

Public fears: Hong Kong's government is legally allowed to request help from the PLA garrison of more than 6,000 soldiers if public order in the city spirals out of control.

There is no indication yet that troops may be deployed -- but the fear of PLA intervention has spread through the city and triggered memories of the brutal 1989 Tiananmen Square crackdown.

What would happen? The stock market would likely crash, followed by the housing market. A mass exodus could follow.

Given Hong Kong's close ties to mainland China, a decision to deploy the PLA could ricochet through the entire country at a time when the world's second biggest economy is already vulnerable because of the trade war with the United States.

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