The United States and Cuba restored full diplomatic relations just after 12 a.m. Monday (0400 UTC) after more than five decades of hostility.
A few hours later in the pre-dawn quiet, the red, white and blue Cuban flag was hoisted at the U.S. State Department, alongside the flags of the other countries that have diplomatic relations with the U.S.
The U.S. and Cuba each now have a full fledged embassy in the other's country.
The historic diplomatic shift comes 54 years after a diplomatic break that happened during U.S. President John Kennedy's administration.
Cuban officials, including Foreign Minister Bruno Rodríguez Parrilla, will attend an opening celebration at Cuba's Washington embassy Monday. U.S. Assistant Secretary of State for Western Hemisphere Affairs Roberta Jacobson, who played a key role in the negotiations between the two countries, will also be in attendance.
U.S. Secretary of State John Kerry will formally receive his Cuban counterpart, Rodríguez Parrilla, Monday. The two officials will hold a joint news conference later in the day.
The opening ceremony for the U.S. embassy in Cuba will be delayed until Kerry can travel to Havana, but the embassy will be fully functional in the meantime.
The re-establishment of ties between the two countries is not without its critics.
Republican U.S. Senator Marco Rubio has said there has not been enough emphasis on Cuba's human rights record. He said earlier this year, "Their view of human rights isn't just different than ours, they are flat-out wrong and immoral in their views."
Roger Noriega, an American Enterprise Institute analyst and a former U.S. ambassador to the Organization of American States, also expressing concern about Cuba's human rights record, said "I think we have had to lower our standards in order to raise our flag in Havana."