Much of central Hong Kong was awash in American flags on Sunday, as tens of thousands of prodemocracy protesters marched peacefully past the US consulate, imploring Congress and President Trump to support their struggle to keep Hong Kong free. Thousands of demonstrators sang the “Star Spangled Banner.” Many held placards promoting the Hong Kong Human Rights and Democracy Act, a bipartisan measure on Capitol Hill to authorize sanctions on Chinese officials who suppress democracy and the rule of law in the city. Other signs directly addressed the US president: “Please liberate Hong Kong,” they pleaded. “Defend our Constitution.”

The Hong Kong protests have so far yielded few concessions from China, which has reneged on its 1997 vow to respect Hong Kong’s autonomy for 50 years. Whether Washington is willing to get more directly involved is an open question. Trump himself has sent mixed signals. Last month, echoing the Beijing party line, he referred to the marches as “riots,” and said it was up to China to deal with them. Yet he has also warned Chinese ruler Xi Jinping against quelling the protests with a violent crackdown.

In my view, the United States should be unambiguous in its support for democracy and human rights in Hong Kong. There is room for reasonable debate about how far to go in backing the protesters or confronting Beijing. But when liberty is being choked off by a dictatorship, US policy should never be one of neutrality.

Barack Obama’s greatest failure in world affairs was his paralysis in the face of atrocious persecution by tyrants. For fear of being “seen as meddling,” he declined to support brave prodemocracy protesters in Iran, or to act when Syria crossed his “red line” and murdered civilians with chemical weapons. George H.W. Bush similarly blundered in 1989 when he refused to utter any word of encouragement for the throngs of Chinese citizens peacefully protesting for more freedom — or any word of condemnation when the tanks rolled into Tiananmen Square, and thousands were killed or maimed.

US leaders who share this reluctance to champion democratic ideals and promote freedom abroad think of themselves as realists, but a short-term focus on security and stability has never been in America’s long-term interest. On the contrary: An explicit policy of expanding liberty and the rule of law ultimately keeps America safer than excusing or accommodating despotic regimes.

But that isn’t the only reason to have a freedom-oriented foreign policy.

Ending repression and upholding democracy in the world are not just uplifting ideas. They are, in a sense, America’s enduring mission. The United States is the only nation in history founded on the conviction that freedom is an inalienable right. No other people has ever cared so deeply about advancing human rights and self-government beyond its own borders, nor done so as successfully.

That is why thousands of Hong Kong protesters sing “The Star-Spangled Banner” as they march for their rights. It’s why Chinese students during the vast prodemocracy upwelling of 1989 constructed a “Goddess of Liberty” modeled on the great statue in New York Harbor. It’s why American flags were flown by demonstrators in Libya during the short-lived “Arab Spring.” It’s why Azerbaijanis clamoring for free elections in 2004 held up posters showing the American president with the words “We Want Freedom.”

The United States doesn’t always live up to its ideals, but the power of those ideals to inspire beleaguered people everywhere never weakens. When John Quincy Adams in 1821 proclaimed the United States “the well-wisher to the freedom and independence of all,” he could not have conceived how much America would do over the next two centuries to make the world freer, safer, and happier. Or that activists on the other side of the globe would take heart from the Stars and Stripes, and from the example of the republic for which it stands.

Our politics and civic discourse are pretty terrible these days. Too many of those we elect to high office are shallow windbags or cynical opportunists. It can be hard, at times, to see 21st-century America as anything but a grave disappointment.

But that sea of US flags on the streets of Hong Kong is a vivid reminder of the abiding significance of America in the world’s imagination. We are still, in the famous phrase, as a city upon a hill. We may lose sight of that at times, but the rest of mankind doesn’t.

(Jeff Jacoby is a columnist for The Boston Globe).