In a landmark visit to Cuba, President Obama toured parts of Havana on Monday, marking the first time in almost nine decades that an American leader has traveled to the Caribbean nation. He met with the island nation's president, Raúl Castro, for talks in the Palace of the Revolution, a site once in the crosshairs of the U.S. military as a target for a potential nuclear strike.

Now, it's the probable launchpad of a new phase in U.S.-Cuba relations. "Change is going to happen here," Obama declared ahead of his meeting with Castro.

The 84-year-old Cuban leader has played a conspicuous role in his nation's recent history and in the dramatic thaw with Washington.

Castro became de facto president in 2006 when older brother Fidel Castro took seriously ill. In 2008, he was officially installed as president. Before that, he had spent a half-century in his brother's shadow, largely as defense minister.

At a press conference in Havana, with President Obama at his side, Cuban President Raul Castro denied his country has political prisoners. (AP)

The two were co-conspirators in the Cuban revolution, and Raúl served in a variety of posts after its triumph in 1959 — the quiet lieutenant to a charismatic, continental firebrand. With his background more rooted in the military, there is little indication that the current Cuban president has much interest in a real democratic transformation.

But since coming to the fore, the younger Castro has proved to be more pragmatic than the bombastic Fidel.

"His style has brought an important shift that has done just as much as anything to dial down hostilities with Washington and set the stage," wrote my colleague Nick Miroff in 2014, after the Obama administration had first signaled its intent to move toward normalization of ties.

Castro cautiously pushed toward changes to Cuba's sclerotic socialist economy. "Many Cubans confuse socialism with freebies and subsidies and equality with egalitarianism," he was quoted as saying by the BBC in 2010. He has moved toward certain liberalizing measures and has encouraged increased, diversified foreign investment. As much as 40 percent of the country's population now earns a living through the private sector.

It's still unclear, though, whether Castro's reform efforts also signal an onset of genuine political change. In his joint news conference with Obama on Monday, the Cuban leader and his American counterpart stressed the enduring, fundamental differences between their countries — from the continued suppression of dissent and detention of dissidents in Cuba to the long-standing U.S. military presence in Guantanamo Bay, Cuba.

Castro has said that he will step down in 2018, probably to make way for a handpicked successor from within the regime.

"There have been few substantive changes in the heart of the sole party with regards to tolerance for diversity of opinions or for [unsanctioned] opposition groups," Jorge Duany, a scholar at the Florida International University Cuban Research Institute, told Agence France-Presse.

But there's a sense that an economic opening to the United States will inevitably change Cuba, where the single-party state isn't as insulated from outside pressures as those in far larger nations such as China and Vietnam.

“In the medium and long term, this is a challenge for the Cuban system, because it undermines the climate of hostility that has long been used to justify the one-party state,” Arturo López Levy, a former Cuban government intelligence analyst, told Miroff in 2014.

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