THE FULL-SCALE assault on freedom in Hong Kong launched this week by China’s Communist Party leadership could mark a fateful turning point in its relations with the democratic world. Until now the regime of Xi Jinping has sought to suppress a protest movement in the former British colony with tear gas, arrests and prosecutions of opposition leaders. Having failed, Beijing on Friday unveiled what amounts to its nuclear option — a new legal framework that would allow mainland security agencies to establish themselves in the territory and enforce a ban on “secessionist or subversive activity, the organizing of terrorist acts” and “activities of foreign and external interference.”

As Hong Kong opposition leaders were quick to point out, the move would effectively gut the “one country, two systems” principle that has allowed the survival of the rule of law and freedom of speech and assembly in Hong Kong since it returned to Chinese sovereignty in 1997. Though technically a national security law was mandated by the constitution Hong Kong then adopted, it will be written and enacted by the rubber-stamp National People’s Congress in Beijing rather than the democratically elected Hong Kong legislature, which has deferred the law since mass protests against it in 2003.

There’s every reason to expect that once the legislation is in place this summer, Beijing’s thugs will employ it to behave as they do on the mainland — crushing dissent by subjecting those who criticize the regime to disappearance, torture and lengthy prison sentences. First in line could be those Hong Kong leaders, such as Martin Lee and Joshua Wong, who have traveled to Washington and other Western capitals to lobby for pressure on Beijing to fulfill its commitments on Hong Kong, including for universal suffrage and free elections. Both are already facing prosecution.

Such a crackdown would compound what is already a crisis in U.S.-Chinese relations and present Washington with some difficult choices. It doesn’t help that President Trump and Secretary of State Mike Pompeo have been heaping abuse on the Xi regime in recent weeks as a way of distracting from the Trump administration’s abysmal response to the covid-19 pandemic; in fact, the mounting U.S. hostility may have persuaded Mr. Xi that he had little to lose by smothering Hong Kong.

On Friday, Mr. Pompeo issued a blistering statement saying the pending People’s Congress action would “be a death knell” for Hong Kong’s autonomy and would “inevitably impact our assessment of One Country, Two Systems and the status of the territory.” That was an unmistakable reference to the special trading privileges Hong Kong has enjoyed under U.S. law since 1992. Under an amendment Congress adopted last year, the State Department must issue a report on whether the territory remains “sufficiently autonomous” to justify the measures, which include exemption from tariffs applied to mainland exports.

If a negative report by the State Department led to a repeal of the privileges, Hong Kong’s economy would be devastated — as would a lot of U.S. businesses. The estimated $38 billion in annual U.S.-Hong Kong trade would be at stake; so would the regional headquarters that some 290 U.S. companies maintain in the city. The result could be to speed the conversion of China’s most free city into just another provincial capital, which is not in the U.S. interest, let alone Hong Kong’s. A more effective response would look something like that proposed by Sens. Chris Van Hollen (D-Md.) and Patrick J. Toomey (R-Pa.), who would sanction Chinese entities that compromise Hong Kong’s autonomy; it would also sanction banks that do business with those entities.

The assault on Hong Kong requires a robust U.S. reaction — but one that is carefully calculated and not driven by election-year demagoguery.

Read more: