Kissinger and Carter

In the mid-1970s, during Gerald Ford's presidency, then secretary of state Henry Kissinger launched in the utmost secrecy what was then the most developed attempt at normalization of relations since they broke off in January 1961.

But the intervention of Cuban forces in Angola in 1975 on behalf of the People's Movement for the Liberation of Angola - which now rules the African nation - sounded the death knell on the effort.

Just a few weeks after taking office in 1977, Jimmy Carter ordered a new round of talks aimed at normalizing ties with Havana.

In the early weeks, things seemed to be moving along - agreements were reached on maritime disputes, commercial fishing and on opening diplomatic interests sections in each country.

"I always had a high opinion of Carter as a man of honor, an ethical man," Castro said years later. "Carter was a man who wanted to fix the problems between the United States and Cuba."

But again, the effort wavered over Cuba's military actions in Africa. Havana refused to budge, and Carter said the time wasn't right to move forward.

Bush and Obama

Fast forward to the turn of the century, and the presidency of George W. Bush for whom only one rule mattered - no concessions without "regime change". So no progress was made.

But in the spring of 2013, as Obama launched into his second term, he authorized the start of exploratory discussions with Havana. The first meeting took place in June that year in Canada.

Pope Francis personally wrote to both leaders - Obama and Fidel Castro's brother Raul, now the president - to urge them to move forward.

In October, US and Cuban delegations found themselves at the Vatican, with officials from the Holy See, to finalize the terms of their normalization of ties.

On Dec 17, 2014, Obama and Castro stunned the world with their announcement that Washington and Havana would resume formal ties.

The nearly unthinkable had happened: top-secret talks had gone on for 18 months without a single leak to the media.

Why so much secrecy? For LeoGrande, there are still powerful reasons to tread carefully on the subject of Cuba - though not the same ones as a generation ago.

"In the 60s and 70s, it was more the issue of the Cold War," he told AFP.

"That's why Johnson decided not to follow up on Kennedy's initiative."

But from the 1980s, the influence - and political clout - of the Cuban-American community in Florida started to weigh heavily on the minds of US political leaders.

"Presidents were afraid - particularly Democratic presidents and Democratic candidates - that if they hinted at an opening to Cuba, they could lose the state of Florida and lose the election," LeoGrande said.