On Friday, John Kerry will become the first US secretary of state to visit Havana, the capital of communist Cuba, since 1945.
His mission is to raise the Stars and Stripes once again over the re-opened US embassy and set the seal on a long-delayed thawing of ties kept on ice through the long Cold War.
His trip to America's island neighbor will only last a few hours but it will allow Kerry to discuss sticking points in the detente process, such as the trade embargo and the status of the US base in Guantanamo Bay.
He will also have a chance to distance himself a little from official Havana and meet nervous dissidents wary of the changes underway.
And he has even promised to take a stroll through the city and meet ordinary Cubans.
"I will take an open, free walk in Old Havana," Kerry told Spanish-language US network Telemundo before his departure.
This will take Kerry away from the imposing concrete and glass US mission on the Malecon seafront -- now redesignated a full embassy -- and into the touristy colonial quarter.
Officials said that after the ceremony at the embassy and the walkabout, Kerry would meet at the US chief of mission's residence with Cuban officials, entrepreneurs, artists, activists and dissidents.
"Normalization is a process. We've been very clear about that," State Department deputy spokesman Mark Toner said.
"Certainly, we'll take another step in that process on Friday with the raising of the flag after 54 years of hiatus.
"We've been very clear that this doesn't alleviate every challenge in the relationship, but it does give us the ability to speak directly with and to the Cuban government."
Washington and Havana reopened embassies in their respective capitals on July 20, 54 years after relations broke down at the height of the Cold War between the West and the Communist bloc.
This was the culmination of a rapprochement that was announced in December last year between President Barack Obama and his Cuban counterpart Raul Castro.
Cuba's foreign minister Bruno Rodriguez was in Washington last month to inaugurate his embassy, and Castro and Obama met in April on the sidelines of the Summit of the Americas in Panama.
The US administration has removed Cuba from its list of "state sponsors of terrorism" -- opening the path for a further lessening of trade and diplomatic sanctions.
But the thaw has not been uncontroversial.
Obama's Republican opponents at home have accused him of easing up on pressure on Cuba too quickly while it is not yet a democracy, and Cuban dissidents used to Washington's support have protested, fearing isolation.
No dissidents at flag ceremony
Meanwhile, the father of the Cuban revolution, Raul's brother and predecessor Fidel Castro -- who celebrates his 89th birthday Thursday on the eve of the visit -- has sounded a less conciliatory note.
Kerry, therefore, will tread a cautious path. He will meet select Cuban opposition figures to reassure them that the United States still supports a pluralist future for their island, while not treading on official toes.
No dissidents will be invited to the embassy ceremony alongside Cuban leaders, for example, but will instead attend the private event at the head of mission's residence.
"There will be a broad cross-section of Cuban society that will be invited to that event at the mission," Kerry told Telemundo.
"What they are not invited to, quite openly, is the raising of the flag at the embassy itself, because that is a government-to-government moment -- with very limited space by the way -- which is why we are having the reception later in the day."
A senior US diplomat insisted that this was not a snub to the dissidents who have braved repression in Cuba for decades to challenge the authoritarian regime now renewing ties with their friends in Washington.
But passions are running high in Havana. Some 90 dissidents, many of them wearing sarcastic Obama masks, were arrested on Sunday at a protest against the reopening of the embassy.
"It's his fault, what's happening," said former political prisoner Angel Moya at the protest, blaming Obama shortly before he was arrested.
"The Cuban government has grown even bolder... That's why we have this mask on. Because it's his fault."
Castro's government, meanwhile, will seize on the visit to push its case for further concessions -- in particular for the complete lifting of the economic embargo the United States imposed on its tiny economy in 1962.
Obama and Kerry support this, but their hands are tied by the 1996 Helms-Burton Act and they will need to convince a US Congress led by their Republican opponents to change the law.
As things stand, this looks like a long shot, with conservative lawmakers determined not to gift a victory to the defiant Castro brothers who have seen out so many US presidents.
But there are signs of shift in US public opinion, even in the notoriously anti-communist Cuban-American community.
"There has been a shift within US public opinion in favor of Cuba," said John Gronbeck-Tedesco, associate professor of American Studies at Ramapo College in New Jersey.
"We have now a Cuban American community that is in favor, by a small margin, of repealing the embargo and by a large margin in favor of diplomatic relationship with Cuba."
After half a century as an unbridgeable gulf, the Straits of Florida may have started to narrow.