CHINA THREW down the gauntlet on Wednesday with the arrest of dozens of pro-democracy politicians and activists in Hong Kong, and not only to those who value freedom in the territory. It was also an insult to the European Union just days after it approved a new investment and trade pact with China, and a direct challenge to President-elect Joe Biden. All of them must respond forcefully to China’s rapid and wrenching destruction of liberty in Hong Kong.

In recent months, the regime of Xi Jinping and its puppets in Hong Kong have targeted for punishment individuals who had become icons of the democracy movement, such as organizer Joshua Wong and newspaper publisher Jimmy Lai; members of the legislative assembly; and a group of youthful activists who attempted to flee on a speedboat.

On Wednesday, the dragnet was much wider. At least 53 people, including former lawmakers, activists and an American lawyer, were arrested by more than 1,000 police under Beijing’s new national security law, imposed last summer, and their offices and homes raided. Their crime? They were either candidates or helped to facilitate a primary ahead of legislative elections that were ultimately postponed by the authorities. It is no accident that China defines one of the most elemental and essential of democratic acts — running for office — as “subversion.” The arrests will undoubtedly chill the ambition of others who want to fight for the open society that Hong Kong once symbolized and was supposed to be preserved under the 1997 handoff to China by the United Kingdom.

Mr. Xi reaped a handsome reward the other day by closing the new E.U.-China deal over the objections of critics who said it did not do enough to uphold labor standards at a time when China is extracting forced labor and abusing human rights in Xinjiang and Tibet. Mr. Xi probably figured with the pact in his pocket, he was free to grind down Hong Kong still more. The European Parliament, however, must still approve the deal, and its members ought to refuse as long as democracy is being sundered in Hong Kong.

Mr. Xi may also have hoped to take advantage of political turmoil in the United States. The right response was that of Mr. Biden’s nominee for secretary of state, Antony Blinken, who said Mr. Biden will “stand with the people of Hong Kong and against Beijing’s crackdown on democracy.” It will be refreshing to see a U.S. president actually care about fighting despotism once again.

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