The occupation of Hong Kong’s international airport by anti-government protesters Monday presents China with an unwelcome choice: allow Hong Kong to move toward full democracy or use its own military to forcibly suppress the protesters. President Trump should not stand idly by if the Chinese government chooses to use force.

Hong Kong was handed over to China by the British in 1997 under an arrangement called “one country, two systems.” The idea allowed Hong Kong to keep its own political and economic system for at least 50 years while handing ultimate sovereignty in matters of military and foreign affairs over to Beijing.

The policy has not caused Hong Kong residents to become friendlier toward the Chinese way of life as its original architects had probably hoped. Instead, young Hong Kong residents who have grown up under “one country, two systems” have become ever more attached to the special administrative region’s distinctiveness. They know they are freer and richer than mainland Chinese, and they want to become more, not less, like a successful Western country.

The protests over the past few months are merely the latest and most intense of a growing tendency toward mass, pro-democracy street protests in Hong Kong. Younger residents know that unless they can make Hong Kong a full democracy, Beijing’s political demands will slowly erode their freedom. They see that they ultimately will become like the mainland Chinese, materially well off but politically and socially unfree, unless they act now.

China, of course, knows it can never permit this. If Hong Kong were to become a fully democratic, wealthy part of China, then other parts of China could demand similar treatment. The ultimate logic of giving Hong Kong more political freedom would be to allow China itself to transition into a Western-style democracy — unthinkable for the Chinese Communist Party.

The “one country, two systems” idea was always based on a flawed premise unless it was only ever intended to serve as a fig leaf for Chinese communist imperialism. There can only be “one country” if there is a shared sense of national identity underlying that country. Where there is not for ethnic reasons, as in Tibet and Xinjiang provinces, Beijing has not hesitated to rule despotically. The regime presumably had hoped that shared Chinese ethnicity would prove to be a unifying factor with Hong Kong residents, but that hasn’t happened.

Instead, Hong Kong’s distinct political and economic liberties have created a new national identity, that of the Hong Kong Chinese. The Hong Kong Chinese are fully Chinese in their culture and history but largely Western in their politics and economics. They are more like the westernized residents of Singapore, Japan or Taiwan than they are like mainland Chinese. For Hong Kong Chinese, China is an “other,” not the mother country.

It is this difference that presents China with its fateful choice. As is becoming clearer by the day, Hong Kong’s residents can no longer be relied on to submit peacefully to Beijing’s rule. If President Xi Jinping intends to have “one country,” he will have to treat Hong Kong’s dissidents the way China treats those in Tibet and Xinjiang — as enemies of the state, who have no rights and no recourse.

Removing the velvet glove to reveal the iron fist could have huge consequences for China. Hong Kong is rich because it cuts its own trade deals and has its own World Trade Organization membership. These advantages, which have encouraged foreign firms to invest billions of dollars in the Hong Kong economy, will have no further basis if China reveals itself to be the land’s true ruler by forcibly suppressing the protesters.

Hong Kong’s separate legal status means that its exports are not currently subject to Trump’s China tariffs. If Beijing intervenes in Hong Kong, Trump should immediately extend his China-targeted tariffs to goods and services imported from Hong Kong. That would cause added economic pain in the United States, as more than 1,300 U.S. firms currently do business with Hong Kong. But it would also prevent China from using Hong Kong to end-run his tariffs while sending a clear signal that the United States still stands behind people yearning to be free.

Abraham Lincoln once said that a house divided cannot stand — that a nation must become all one thing or all another. “One country, two systems” was an attempt to refute that thesis, but Beijing is rapidly learning that Lincoln was correct. That’s why China cannot let Hong Kong become free — and why the United States must act when the moment comes.

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