After calling for freedom and democracy as he stood alongside the Cuba's Communist leaders, Obama has touched down in another Latin American nation with a history of delicate relations with Washington.
After a series of historic but at times awkward public appearances with Cuba's Communist leader Raul Castro, Obama will meet Argentina's new free market-friendly President Mauricio Macri.
Yesterday's deadly bomb blasts in Brussels prompted Argentina to put its security forces on high alert as it prepared for Obama's visit.
But the delicate issue of US involvement in Latin America's violent history will rear its head during his visit to Buenos Aires -- after the Havana visit touched on sensitivities over human rights in Cuba.
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Tomorrow morning Obama will pay homage to victims of the "dirty war" by Argentina's dictators against dissidents.
Thursday marks the 40th anniversary of the military coup that started the 1976-1983 dictatorship. Declassified documents have shown that top US officials backed the coup.
Later in the day he holds talks with Macri, lays a wreath at Buenos Aires Metropolitan Cathedral and meets with local people before attending a state dinner.
As well as becoming the first US president to visit Cuba in almost a century, Obama hopes to remake the United States' image in Latin America, tarnished by involvement in coups and death squads.
Obama's administration said last week it would declassify military and intelligence records linked to Argentina's "dirty war."
The documents may shed more light on US involvement in "Operation Condor," a plan among secret police agencies across South America to target leftists and dissidents.
The sensitive date of the Argentina visit angered some victims' groups. Several groups have called on Obama to apologise for US support of the military regime.
But four opinion polls showed a majority of Argentines approved of Obama's visit.
Obama "believes that part of moving forward in the Americas or any other part of the world involves a clear-eyed recognition of the past," said Ben Rhodes, one of the president's top advisers.