Cuban flags fly beside the United States Interests Section in Havana. (Enrique de la Osa/Reuters)

President Obama and Cuba's Raul Castro met on April 11 on the sidelines of the Summit of the Americas, and the two leaders agreed to foster 'a new relationship' for the first time since it was severed by Washington in 1961.

Diplomats from both nations have met for three rounds of talks, and Obama is expected to make a formal announcement any time that would set a process in motion to clear away what has been the biggest sticking point — Cuba's presence on a U.S. blacklist of "state sponsors of terrorism."

[Related: After 33 years, the U.S. is dropping its claim that Cuba sponsors terrorism. Here’s what it means.]

Once Obama and Castro have a deal in place, U.S. officials say the two countries would move quickly to formally restore relations and reopen embassies in Havana and Washington.

Secretary of State John Kerry would most likely travel to Cuba for the ceremony. Depending on the outcome of Saturday's talks, it could happen in a matter of weeks.

How will that work?

The United States doesn't need to build a diplomatic mission in Havana. It already has one.

Keep in mind that the two countries are more like intimate antagonists — an estranged couple — than true enemies. And there may be no better symbol of this than the massive U.S. diplomatic compound along Havana's seafront boulevard, completed in 1953.

After relations were severed in 1961, American officials mothballed the six-story modernist structure with sweeping views of the sea. U.S. officials returned in 1977 when the two countries opened "Interest Sections" in Washington and Havana at their old embassies, under the auspices of the Swiss government.

When the U.S. officials walked in and dusted off the desks, there were still photos of President Kennedy and calendars from 1961 on the walls, recalls The Post's Karen DeYoung, who covered the U.S. diplomats' return.

So the American presence in Havana is tiny, right?

Hardly. Despite lacking true embassy status, the American compound is the largest foreign mission on the island, with about 50 U.S. staffers and more than 300 Cuban employees.

Its current chief of mission, Jeffrey DeLaurentis, does not have ambassador status in the country, but everyone — including some Cuban officials — call him that anyway, and he has held a number of high-level State Department positions. He lives in a separate, palatial U.S. residence, built in 1941, where U.S. diplomats host cultural events and receptions.

Any potential future U.S. ambassador to Cuba would need Senate confirmation, but if lawmakers such as Sen. Marco Rubio (R-Fla.) block Obama's nomination, DeLaurentis could remain as charge d'affaires.

So it's just a matter of changing the sign on the building?

Not quite. One of the friction points in the recent U.S.-Cuba talks has to do with Cuba's restrictions on American diplomats. Both countries limit the movement of each other's personnel, and the Cuban government objects to American meetings with the island's dissidents and opposition groups.

Contrary to some reports, U.S. officials say they are not asking the Cuban government to give them carte blanche to travel the island freely. Rather, they're looking to have the same sort of arrangement that they have with one-time antagonists such as Vietnam, in which U.S. officials can travel widely but provide formal diplomatic notice before doing so.

The U.S. is also looking for Cuba to lift restrictions on diplomatic staffing and on its ability to import equipment into the country, or even construction materials to repair and modernize the building.

Washington has also asked the Cuban government to reduce the heavy police presence around the U.S. compound, where a cordon of stoic Cuban agents in wrap-around sunglasses stand every few yards under the broiling sun.

Havana says the security detail is there to protect the U.S. mission, but they also have the effect of intimidating ordinary Cubans from approaching.

So the building is pretty much walled off?

Not at all. The number of Cubans applying for U.S. visas — either to visit or emigrate — is massive. Every day, hundreds line up before dawn in a nearby park to wait for appointments, and last year, the United States issued more than 40,000 visas through its Havana mission. It turned away several times as many.

The Interests Section also has a consulate for American citizens who are visiting the island or reside there. And it has a computer lab offering free Internet access to visitors.

Is the building still a target for Cuban government protests?

One of the things that has arguably helped make detente possible is that Raul Castro has turned down the heat in the U.S.-Cuba relationship, a clear departure from his brother's political style.

Fidel Castro made the U.S. compound a focus of his confrontation with Washington, leading huge protest marches past the building where Cubans would chant "anti-imperialist" slogans. They peaked during his campaign to bring home child castaway Elian Gonzalez in 2000.

When the George W. Bush administration placed a large scrolling display outside the building a few years later, the Cuban government responded by obstructing the view with a sea of black flags.

Like the ticker, they were taken down, and the flagpoles in front of the U.S. building now fly Cuban flags. Raul Castro's government hasn't staged a protest march outside the U.S. mission in years.

So what's so important about having a U.S. embassy in Havana?

By reopening the building as a formal embassy, Obama officials say they are sending an important signal that their Cuba thaw is real and not easily reversible.

It will give confidence to American tourists and businesses interested in visiting the island, or trying to take advantage of new trade opportunities despite remaining U.S. sanctions, they say.

And it represents a new era of Cuban engagement, they assert, opening the formal diplomatic channels that the U.S. uses elsewhere for expressing its views and attempting to exert influence, just as in China, Vietnam and other countries where its relations are strained.

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