A little more than a year ago, Cubans were exulting in the normalization of relations between their country and the United States after decades of trade and travel barriers. But on a recent trip to Havana, I found Cubans dismayed by President Trump’s reversal of the détente started by his predecessor, Barack Obama.

Mr. Trump had appealed to older Cuban-Americans by taking a tough stance against the Castro regime on the campaign trail, and they helped him carry the Florida in the 2016 election. In August, reports surfaced of American and Canadian diplomats in Havana experiencing mysterious ailments that resulted in hearing losses and cognitive problems. The illnesses, which the United States government has suggested were caused by a “sonic weapon,” appear to have given Mr. Trump the pretext to fulfill his campaign promise and take a harder line on Cuba.

The result is a return to a Cold War mentality and a set of failed policies that will do little to improve human rights in Cuba or to hasten the end of the Castro regime. The embargo imposed by the United States for more than 55 years only strengthened Cuba’s authoritarian government by restricting people’s access to opportunities to better their lives. With a return to Cold War-era policies, it is the Cuban people — not their government — who will suffer.

Nearly two dozen American diplomats returned to the United States for medical treatment for ailments that included dizziness and nausea. The cause remains a deep mystery. Mr. Trump, providing no evidence, said he believed that the Cuban government was responsible. Yet many experts say it is unlikely that a sonic device could have caused the variety of symptoms experienced by the diplomats. The Cuban government has denied involvement in an attack.