President Trump’s speech in Miami this afternoon will go down as one of the remarkable moments of his presidency. He — and a man with a violin — retrieved the cause of Free Cuba that had been abandoned by President Obama in a deal with the Castro regime. He stood not only with the people of Cuba and of Little Havana but with the Congress of the United States. He did so by bowing to the law, known as Helms Burton, which sets the conditions for normalization.

Any changes to our relationship, Mr. Trump said, would be conditioned on “real progress.” He said that the “new policy” — actually, long-standing policy until it was abandoned by President Obama — “begins with strictly enforcing U.S. law.” He vowed: “We will not lift sanctions on the Cuban regime until all political prisoners are freed, freedoms of assembly and expression are respected, all political parties are legalized, and free and internationally supervised elections are scheduled.”

There should be nothing divisive or troubling about those conditions. They are among the conditions outlined in black-letter American law, known as Helms Burton, about which we wrote this morning, among other occasions. Until such conditions are met, he said America would enforce the embargo and the ban on tourism. “When Cuba is ready to take concrete steps to these ends, we will be ready, willing, and able to come to the table to negotiate that much better deal for Cubans, for Americans,” the President said.

Why President Obama saw fit to freelance a new policy and travel to Havana to sup with Raul Castro is a mystery to us. It’s hard to figure which is more shocking, though — his policy or his preparedness to proceed on such a policy in violation of the black letter of the Libertad Act, which is one of the names for Helms Burton. It is of a piece with his preparedness to sign a climate accord that he couldn’t get through Congress and an Iran deal the Congress pointedly opposed.

This is something to think about for those who are horrified at the divisiveness of our politics today. The course upon which Mr. Trump has just set is one that was written in the Congress and passed by a vote of 74 to 24 in the Senate and 336 to 86 in the House. It was signed by President Clinton. It is properly codified United States law, achieved in a bipartisan fashion the absence of which is being mourned today by everyone from Nancy Pelosi to Ted Cruz.

No doubt there are those who will say that times have passed the law by. In a democracy, though, that can’t mean abandoning the law without action in the legislature. The fact is that at no time in recent years has the Congress been prepared to change Helms Burton absent the kind of real, verifiable progress to which the law itself requires the president to attest in writing. It is just terrific to see a president stand on these bedrock principles.

And then Mr. Trump set, so to speak, it all to music. This was his ending anecdote about the famed violinist Luis Haza, whose father, chief of police in Santiago de Cuba, had been one of 71 Cubans whom Castro’s thugs executed by a firing squad near San Juan Hill. “Luis,” Mr. Trump told his audience at the Manuel Artine Theater in Little Havana, turned to the violin and “buried his grief in his great love of music.”

He learned to play so beautifully that, as Mr. Trump told the story, the regime sought to dragoon him into playing for propaganda. Castro’s soldiers barged into his orchestra practice area to fetch the lad, then 12 years old, but Young Haza refused to go. Brandishing their guns, they ordered him to play. The lad picked up his fiddle and played the Star Spangled Banner. We think of Colonel Bud Day, who was awarded the Medal of Honor for, among other things, breaking into our anthem when guards held a gun to his head in the dungeons of Vietnam.

Then President Trump reported that Mr. Haza, now a grown man, was in the Manuel Martine Theater and invited him to the stage to play. So Mr. Haza gave us the same Spangled Banner that he gave the Castros’ goons. Presidential moments don’t get much better, and we’d like to think the anthem in Miami will lift the hearts in a country riven over politics and immigration and looking for signs of hope and grounds for optimism.