Washington - The renewal of full diplomatic relations between the United States and Cuba could be the start of a long thaw in decades of animosity but the path is full of obstacles, not least the US sanctions on the Communist-run island.
The embargo is enshrined in law, most notably the Helms-Burton Act of 1996, which tightened bans on US trade with Cuba and Americans visiting the country.
Only Congress can overturn the law, and with Republicans due to take control of both the House of Representatives and the Senate in January, the chance of lawmakers scrapping all sanctions on Cuba soon is almost zero.
The Obama administration acknowledged that Congress, already upset at the Democratic president for acting alone to change immigration policy, is in no mood to help him lift the embargo on Cuba.
Also Read: Fidel Castro wins China's Confucius Peace Prize
"We support efforts to remove those restrictions. However, we understand that Congress is unlikely to take those steps in the immediate future," a senior administration official said.
Almost all trade with Cuba is banned although exports of US medical supplies and agricultural products are allowed if approved by the commerce department.
Instead of going through Congress, President Barack Obama will use his executive authority to ease restrictions on some trade, travel and banking ties to allow Cubans to send more money home.
The first concrete step in the new policy is the prisoner swap, under which American aid worker Alan Gross was also freed, arriving at a US Air Force base near Washington on Wednesday. An unidentified U.S. intelligence agent was swapped for three Cuban prisoners held in the United States.
As part of the thaw, the state department is expected to then declare that Cuba is no longer a "state sponsor of terrorism" and remove it from a list that includes Iran, Sudan and Syria. This step does not need congressional approval.
While Cuba supported leftist rebels in the Cold War, few in the US government believe Havana plays a major role in sponsoring terrorism now. In fact, Cuba has hosted peace talks between the Colombian government and FARC guerrillas this year.
"Terrorism has changed in the last several decades. At a time when we are focused on threats from al-Qaeda to ISIL, a nation that meets our conditions and renounces the use of terrorism should not face this sanction," Obama said in a televised speech from the White House.
As tension between the United States and Cuba eases, the two countries will try to re-open embassies in each other's capitals after decades of broken diplomatic relations.
That step might hit a snag if Republicans in Congress attempt to halt funding to the state department for a mission in Havana.
The Senate could also refuse to confirm any ambassador to Cuba named by Obama, although Washington could still operate an embassy run by a more junior diplomat.
Senator Marco Rubio, one of the Republican Party's most influential voices on Cuba, vowed to oppose Obama's new course once the new Republican-controlled Congress begins in January.
"I intend to use my role as incoming chair of the Senate Foreign Relations Committee's Western Hemisphere subcommittee to make every effort to block this dangerous and desperate attempt by the president to burnish his legacy at the Cuban people's expense," he said.
The United States has dropped its objections to Cuba attending a Summit of the Americas in Panama next April.
Sanctions may take years to lift fully, even if Cuba cleans up its human rights act. The White House said on Wednesday it wants to see the embargo lifted by the time Obama leaves office in 2017.
But the Helms-Burton Act states that the sanctions cannot be ended until Cuba transitions to a democratic government that does not include current Cuban President Raul Castro and his brother, former president Fidel Castro.
Also Read: Cured Ebola doctor returns to Cuba
Barring unexpected events, Raul Castro is likely to stay in office for several years more. He has announced he will not seek re-election in 2018.
In addition, the US Congress will be in the hands of Republicans until at least after elections in 2016, so the appetite on Capitol Hill for ending the embargo will be limited.
If sanctions are lifted eventually, there is one Cuban grievance that is likely to remain an irritant: the US presence at Guantanamo Bay Naval Station in southeastern Cuba.
The United States has leased the land where the naval base is located since 1903, although Cuba has not accepted payments for decades.
Guantanamo is now home to the US prison for detainees in the "war on terror," which Obama has failed to close despite promising to do so.