WASHINGTON — The United States and Cuba restored full diplomatic relations Monday after more than five decades of frosty relations rooted in the Cold War.

The new era began with little fanfare when an agreement between the two nations to resume normal ties on July 20 came into force just after midnight Sunday and the diplomatic missions of each country were upgraded from interests sections to embassies. When clocks struck 12:00 in Washington and Havana, they tolled a knell for policy approaches spawned and hardened over the five decades since President John F. Kennedy first tangled with youthful revolutionary Fidel Castro over Soviet expansion in the Americas.

Without ceremony around 4 a.m. ET, maintenance workers placed the Cuban flag in the lobby of the State Department alongside those of other nations with which the U.S. has diplomatic relations.

A worker at the State Department adds the Cuban flag between Croatia and Cyprus ones inside the main entrance in Washington, D.C., at 4 a.m. ET on Monday. PAUL J. RICHARDS / AFP - Getty Images

The historic shift will be publicly memorialized later Monday when Cuban officials formally inaugurate their embassy in Washington and Cuba's blue, red and white-starred flag will fly for the first time since the countries severed ties in 1961. Secretary of State John Kerry will then meet his Cuban counterpart, Bruno Rodriguez, and address reporters at a joint news conference.

The U.S. Interests Section in Havana plans to announce its upgrade to embassy status in a written statement on Monday.

And yet, though normalization has taken center stage in the U.S.-Cuba relationship, there remains a deep ideological gulf between the nations and many issues still to resolve. Among them: thorny disputes such as over mutual claims for economic reparations, Havana's insistence on the end of the 53-year-old trade embargo and U.S. calls for Cuba to improve on human rights and democracy. Some U.S. lawmakers, including several prominent Republican presidential candidates, have vowed not to repeal the embargo and pledged to roll back Obama's moves on Cuba.

Still, Monday's events cap a remarkable change of course in U.S. policy toward the communist island under President Barack Obama, who had sought rapprochement with Cuba since he first took office and has progressively loosened restrictions on travel and remittances to the island.