President Barack Obama is in Cuba for a historic three-day visit to the island and talks with its communist leader.
He is the first sitting US president to visit since the 1959 revolution, which heralded decades of hostility between the two countries.
Mr Obama said change would happen in Cuba and that Cuban President Raul Castro understood that.
The two leaders are due to meet later on Monday to talk about trade and to hold a joint news conference.
For a US president to touch down at Jose Marti airport in Havana and be warmly greeted by Cuban foreign minister was until recently unthinkable.
For decades, the US and Cuba were engaged in a bitter stand-off, triggered by the overthrow of US-backed Cuban dictator Fulgencio Batista by Communist leader Fidel Castro in 1959.
The US broke off diplomatic relations and imposed a trade embargo.
But President Obama undertook two years of secret talks which led to the announcement in December 2014 that the two countries would restore diplomatic relations.
Since then, there have been a series of symbolic moments, such as the first formal meeting of Presidents Obama and Castro at a regional summit in Panama and the opening of embassies in Havana and Washington DC.
The first stop on President Obama’s tour was the newly re-opened US embassy in Havana, where he told staff it was “wonderful to be here”.
But it was the visit to Havana’s old town which brought home the long way US-Cuban relations have come since the thaw was announced 15 months ago.
While the plan to interact with Cubans in the streets was disrupted by a tropical storm, the image of the US president and his family braving the rain demonstrated to many Cubans his commitment to the new, warmer relationship.