How sad, I’ve been thinking, that the founder of Human Rights Watch, Robert Bernstein, a champion of human rights in China, isn’t alive to see the broadcasts of hundreds of thousands, or maybe even millions, of protesters taking to the streets of Hong Kong to defend what is left there of freedom and the rule of law.

What a good sign it is for America, even on matters that, like the tariff fight with China or the rising tensions with Iran, may seem only tangentially related to whether residents of Hong Kong are subject to extradition for trial in mainland China.

The pressure on the Chinese government that is coming from Hong Kong only increases American leverage in the prolonged trade negotiations. China has been a vote protecting Iran at the U.N. security council. China, according to Reuters, is also Iran’s largest oil customer. If China’s regime is pinned down or preoccupied with the Hong Kong protests and their internal ramifications, it may be less willing to endure the economic costs of an extended trade battle with the Trump administration or a clash with America about Iran.

There is some risk that China might pick a fight with the United States as a way to distract from the Hong Kong situation and rally nationalistic Chinese around the flag. But that could backfire badly for the Chinese Communist regime.

The positive nature of the news from Hong Kong, though, goes way beyond mere short-term or medium-term negotiating advantage for America. It has to do with the chance that the protests, if successful, might help restore American idealism about the chances for the spread of freedom, democracy, and the rule of law around the world.

That optimism peaked with the defeat of the Soviet Union and its client states, and it carried through the overthrow of Saddam Hussein and the early days of the Arab Spring. It dissipated, though, with events in the Middle East — Bashar al-Assad’s crushing of the Syrian opposition, the postwar chaos in Iraq, the rise of the Islamic State, persistent authoritarianism in Turkey, Egypt, and Saudi Arabia. Whatever the explanation — poor American leadership, a lack of institutional or cultural preconditions — the spread of freedom lost the sense of inevitability that it had acquired in the 1990s and early 2000s.

Relatedly, Americans lost the sense that they could force the issue. Mr. Trump gets bashed in some circles for cozying up to dictators like President Putin of Russia, but in some ways (importantly, not in all ways, as Benjamin Netanyahu can attest, but in some ways) Mr. Trump’s foreign policy has been less of a radical departure from Barack Obama’s than a continuation of President Obama’s post-Iraq-War humility about the use of American force to advance idealistic objectives.

In fairness to the architects of the Iraq War, that intervention was never solely idealism about ending a dictatorship but was also advocated as a post-September 11, 2001, reaction to the threat of terrorists with weapons of mass destruction.

No one — even my most hawkish friends — is openly talking about invading China, or even Hong Kong. What is heartening is the willingness of Americans across the political spectrum to stand up, at least rhetorically, for this distant cause.

Vice President Biden tweeted, “The extraordinary bravery shown by hundreds of thousands in Hong Kong, marching for the civil liberties & autonomy promised by China is inspiring. And the world is watching. All of us must stand in support of democratic principles and freedom.”

Now, it’s fair to ask where Mr. Biden was when Mr. Obama was letting President al-Assad slaughter his opponents in Syria. Where was Mr. Biden when Mr. Obama was providing the Iranian regime with hundreds of billions of dollars in sanctions relief? But it’s great to see a Democratic presidential candidate rising to the issue now.

Mayor Pete Buttigieg also tweeted, “Inspiring to see so many in Hong Kong marching peacefully this weekend. We must continue America’s commitment to Hong Kong’s openness, democratic values, and judicial independence.”

Who would have imagined that it would take the people of Hong Kong to rouse Americans out of our post-Iraq War cynicism about the potential for America to play a positive international role? Who would have predicted that Andrei Sakharov, Lech Walesa, and Natan Sharansky could have defeated the Soviet Union and roused Americans out of our post-Vietnam War doldrums?

Robert Bernstein, it occurs to me, could have imagined it. He died last month at age 96. If he were still with us, he’d surely be smiling at the courage on display in the streets of Hong Kong, and at the way it is resonating also here in America.