The United States and Cuba will re-establish diplomatic ties and open their long-shuttered embassies on July 20, the Cuban government said Wednesday.

Cuba’s foreign ministry made the announcement after the U.S.’s top diplomat in Havana delivered a letter from President Obama to Cuban President Raúl Castro.

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The move is the biggest step yet in Obama’s push to end hostilities with Havana that date back to the Cold War, which he announced in December with Cuban President Raúl Castro.

The Obama administration cleared the biggest obstacle to the shift after it formally dropped Cuba from the list of state sponsors of terrorism in May.

But U.S. and Cuban negotiators still had to work through thorny issues, such as freedom of movement for American diplomats in Cuba and their ability to speak with people outside of the government.

Anticipation for the move built earlier this month after flagpoles were installed at the U.S. and Cuban interest sections, which have served as the countries’ informal diplomatic missions.

Obama and Castro agreed last December to begin restoring long-broken ties between their two countries.

The two leaders met in April in Panama, the most significant interaction between U.S. and Cuban leaders in five decades.

Ending the policy of isolation against Cuba is a major foreign policy goal for Obama, who was elected president promising to use to diplomacy to change the U.S.’s relationship with its allies and enemies alike.

But many obstacles remain. Leaders in Congress are opposed to lifting the U.S. trade embargo against Cuba and U.S. officials continue to voice concern about Cuba’s human-rights record.

Sen. Marco Rubio Marco Antonio RubioSunday shows preview: Justice Ginsburg dies, sparking partisan battle over vacancy before election Florida senators pushing to keep Daylight Savings Time during pandemic Hillicon Valley: DOJ indicts Chinese, Malaysian hackers accused of targeting over 100 organizations | GOP senators raise concerns over Oracle-TikTok deal | QAnon awareness jumps in new poll MORE (R-Fla.), a 2016 presidential candidate, has pledged to block any ambassador nominee from getting a confirmation vote.

Critics of the administration’s Cuba policy condemned the move, citing the Cuban government’s ongoing suppression of political dissidents.

“There was little doubt that the Obama administration would pursue its goal of opening an embassy in Cuba no matter the sad reality on the ground,” said Rep. Ileana Ros-Lehtinen (R), a Cuban-American who represents the Miami area. “Opening the American embassy in Cuba will do nothing to help the Cuban people and is just another trivial attempt for President Obama to go legacy shopping.”

This story was updated at 10:27 a.m.