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The last time President Barack Obama and Cuban President Raul Castro met face-to-face was in December 2013 — for a three-second handshake during Nelson Mandela's memorial service.

The two now could now cross paths again at the two-day Summit of the Americas in Panama, an encounter rife with opportunity and symbolism given the thaw in U.S.-Cuba relations announced at the end of last year.

Obama and Castro spoke by phone Wednesday ahead of the possible in-person encounter at the summit, a White House official told NBC News,

The U.S. for decades had blocked Cuba from attending the meeting, and Friday's start to the two-day summit marks the first time a Cuban leader will be present.

"That hopefully marks the beginning of the end of the Cold War in the Caribbean, in some ways the last vestiges of an era — a bygone era," said Cuba expert Peter Kornbluh, who is attending the summit and co-authored the book, "Back Channel to Cuba." "This summit will go down in history as a turning point in U.S.-Cuban relations and U.S. relations for the region."

Obama set a positive tone ahead of the summit, saying Thursday that a decision is near on whether to remove Cuba from the U.S.'s list of terrorism-sponsored countries.

A formal meeting between Obama and Castro at the annual gathering of Western Hemisphere nations would be the highest level of interaction between the U.S. and Cuba in more than half a century.

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While it was unclear if Obama and Castro will schedule a formal meeting, the two are widely expected to share at least a handshake.

Kornbluh said the presidents could potentially announce the complete restoration of normal diplomatic relations — or at least signal when exactly that could happen. Cubans have been waiting for financial sanctions and other trade restrictions to be lifted.

"President Obama has been up against pretty substantive opposition to rewriting a new chapter in Cuba policy. He needs to consolidate the decisions that he has made to move forward to repudiate the past and pursue a future of engagement," Kornbluh said. "That's why you would assume he'll use the summit to maximum benefit."

A normalized relationship between the U.S. and Cuba would mean shedding the decades of isolation and a trade embargo imposed during the regime of Fidel Castro, the older brother of Raul Castro.

Cuban President Fidel Castro shaking hands with American vice-president Richard Nixon during a press reception in Washington. Keystone / Getty Images, file

The last face-to-face talks between an American president and a Cuban leader were in Panama in 1956, when President Dwight D. Eisenhower met with dictator Fulgencio Batista.

But three years later, Batista was ousted by Fidel Castro and his communist forces as they came to power — troubling the Americans. On April 15 of that year, Fidel Castro visited the U.S. on an invitation from the American Society of Newspaper Editors. He managed an audience with Vice President Richard Nixon after Eisenhower expressed no interest in sitting down with him.

The relationship deteriorated as the U.S. moved to support Cuban exiles who were not in line with Castro's communist regime. The beginning of embargoes imposed by the U.S. in 1960 only ensured the door between the two countries would be closed once President John F. Kennedy took office the following year.

It was a far cry from the last time a U.S. president even visited Cuba while in office: Calvin Coolidge went in January 1928 to address the Sixth Annual International Conference of American States in Havana.

Kornbluh said he imagines this new phase of the relationship tearing down the walls.

"I hope President Obama goes to Cuba himself before his administration ends," Kornbluh said. "I would like to think that Fidel Castro will arrive with a special box of Cohibas."

IN-DEPTH

— Erik Ortiz