The other day, President Trump took it into his head to buy Greenland. That stirred outrage and anger. “Thankfully,” said Danish Prime Minister Mette Frederiksen, “the time where you buy and sell other countries and populations is over.”

Yes, we’ve moved on to a more enlightened era. Take Hong Kong – which is what the Chinese Communist Party intends to do. Back in the last century, the British did not sell that colony and its population. Instead, they gave both away to the regime that by force of arms had conquered mainland China in the aftermath of World War II.

There was a quid pro quo: The People’s Republic of China (PRC) promised that for 50 years the people of Hong Kong would retain the freedoms to which they had grown accustomed. This was called a “principle” and it was given a name: “one country, two systems.”

President Xi Jinping is now reneging on that promise. Hong Kongers have responded with massive and prolonged protests, taking to the streets by the millions, risking everything in an effort to defend their distinct, dynamic and endangered culture and way of life.

“Thankfully, the time where free peoples are subjugated by totalitarians is over,” Ms. Frederiksen didn’t say in response.

Other leaders of what we used to call the Free World have been similarly silent or have uttered diplomatic mush. A joint statement from the European Union and Canada was typical: “It is crucial that restraint be exercised, violence rejected and urgent steps taken to de-escalate the situation. Engagement in a process of broad-based and inclusive dialogue, involving all key stakeholders, is essential.”

A question to which I’ve never received an adequate answer: Before agreeing to turn Hong Kong over to the Chinese communists in 1997, why didn’t Great Britain, backed by its democratic allies, make a serious attempt to hold a plebiscite – to give the people of Hong Kong an opportunity to decide their future for themselves?

China’s rulers might have been reminded that, when they were admitted to the United Nations, they undertook certain obligations. In particular, the 1960 Declaration on Colonial Independence declares: “All peoples have the right of self-determination.” It adds that, “by virtue of that right they freely determine their political status and freely pursue their economic, social and cultural development.”

The bosses in Beijing would not have been pleased but they were weaker and poorer then. Not until 2001 was the PRC admitted to the World Trade Organization (WTO) with special privileges as a “developing nation.” Trade with America and Europe has enriched them since. Foreign policy scholars were confident that increasing commerce and wealth would lead to liberalization. It was a nice theory while it lasted.

At this point, wouldn’t it be inspiring if U.N. Secretary-General António Guterres were to propose a free and fair vote so that Hong Kongers could exercise their acknowledged human right to self-determination? Britain and France, and other democratic members of the U.N. (Denmark, perhaps) could back him up.

What about the U.S.? Though President Trump should be clear in his support for the people of Hong Kong, it would be best if the “international community” were to take the lead here. One reason: The U.S. should never call for outcomes it does not seriously intend to achieve. The U.N., by contrast, does little else.

What’s more, the Chinese-American relationship has become complicated. President Trump, unlike his predecessors, has been attempting to address Beijing’s violations of the WTO, its chronic theft of American intellectual property, its continuing military buildup, its encroachments on freedom of the seas, and its abuses of the most fundamental freedoms.

Mr. Trump has not consistently hit the right notes either on China or Hong Kong but he was correct to recently warn that a trade deal with the rulers on the mainland “would be very hard” to achieve “if they do violence. If it’s another Tiananmen Square … I think it’s a very hard thing to do if there’s violence.”

Not for a minute do I kid myself that Mr. Xi will actually decide to respect the rights of the people of Hong Kong. But if a proposal for a plebiscite were to become part of the debate, he would save face by merely strategically retreating to the “one country, two systems” pledge. Hong Kongers would then have a few more decades to try to work out a better deal than the one they’ve been saddled with.

The converse also is true: If the West meekly accepts the PRC’s subjugation of Hong Kong – as we have meekly accepted Chinese communist subjugation of Xinjiang (where about a million Uighurs have been incarcerated in concentration camps) and Tibet (which Beijing is in the process of colonizing), Mr. Xi may be encouraged to next seize (not buy) the free and democratic nation of Taiwan. That would have enormously negative ramifications in Asia and beyond.

One last thought: Greenland became a Danish colonial possession in the 18th century. In the 20th century Denmark decided to call the island an “autonomous constituent country” – a prettier term. Geographically, however, Greenland is part of North America, and most Greenlanders are Inuit – closely related to Inuit of Canada and Alaska.

The people of Hong Kong have never had an opportunity to voice their preferences about their future at the ballot box. Prime Minister Frederiksen might consider doing better by the people of Greenland.

Clifford D. May is founder and president of the Foundation for Defense of Democracies (FDD) and a columnist for the Washington Times.