THE SENTENCING of activists Joshua Wong, Agnes Chow and Ivan Lam in Hong Kong for taking part in pro-democracy demonstrations last year should alarm everyone concerned about the retreat of democracy and rule of law. China is rapidly drowning Hong Kong in the mainland’s intolerant authoritarianism. As the prison cell doors clang shut on these three, another piece of Hong Kong’s treasured liberty will be lost.

Mr. Wong was a teenager when he emerged as a protest leader in Hong Kong, successfully rallying students to oppose the Chinese Communist Party’s indoctrination in schools. His years of activism since have made him an icon of the pro-democracy fight in Hong Kong, the former British territory turned over to China in 1997 under an agreement that it would preserve its autonomous government, freedoms of speech and assembly, and the rule of law for 50 years. China has now reneged on that agreement, most egregiously with a new national security law that has become a cudgel against dissent.

Mr. Wong, sentenced Wednesday to 13½ months in prison, had used a megaphone during the 2019 protests over the national security law, chanting with the crowd, “No riots, only tyranny!” That is now the kind of speech China has outlawed. The court said it was evidence of “incitement to knowingly take part in an unauthorized assembly.” Mr. Lam, the son of a police officer, was sentenced to seven months, and Ms. Chow to 10 months. They were key figures in the 2014 Umbrella Movement, a street occupation that called for Hong Kongers to be allowed direct election of their leaders. When China refused, they formed a pro-democracy group, Demosisto, which they recently folded in face of restrictions in the national security law.

Would China be a bit more careful if President Trump had been less erratic and more forceful in standing up for democracy? Would Mr. Wong be free if the president hadn’t signaled to Chinese leader Xi Jinping privately at the Osaka, Japan, summit in 2019 that the United States would tone down criticism on Hong Kong to revive trade talks? Would Mr. Xi have been deterred if Mr. Trump had given a speech saying China “runs against the tide of history by denying human freedom and human dignity to its citizens,” which it does — and is how President Ronald Reagan described the Soviet Union in 1982? We won’t know because Mr. Trump chose to favor dictators over democracy.

President-elect Joe Biden has a golden opportunity when he takes office in January to show China that in the multiple channels of its relationship with the United States, democracy and human rights will remain central, not just be trotted out for an election year. Whether Mr. Xi likes it or not, Mr. Biden should repeat the names — Joshua Wong, Agnes Chow and Ivan Lam — from the very start of his engagement with China.

Mr. Biden might also remind Mr. Xi of the wise and enduring message in Mr. Wong’s first letter from prison: “Cages cannot lock up souls.”

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