WASHINGTON (REUTERS) - 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 companies trading 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.
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.
US banks will be allowed to open correspondent accounts at Cuban banks, and trade with Cuba's private sector will be easier.
The first concrete step in the rapprochement 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 US 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 leftist FARC guerillas this year.
As tension between the two countries eases, the United States and Cuba will then try to re-open embassies in each other's capitals after decades of broken diplomatic relations.
That step might hit a snag if Obama names a US ambassador to Cuba because the appointment would require Senate confirmation, meaning that Republicans could block it.
Washington could operate an embassy in Havana without an ambassador, leaving it in the hands of other senior diplomats.
Sanctions may take years to lift fully, even if the White House wants to and Cuba cleans up its human rights act.
The Helms-Burton Act states that the embargo 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.
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 in his first presidential election campaign in 2008.
