The face-to-face conversation between President Obama and his Cuban counterpart, Raul Castro, marked the most significant thaw in the two countries' relationship in decades. The meeting, held Saturday at the Summit for the Americas, signaled that the normalization of relations with Cuba, which President Obama announced last December, is proceeding. In the coming days, the U.S. State Department is expected to mark another tangible sign of progress: removing Cuba from its list of state sponsors of terrorism.

However, the process of fully restoring ties will be long and difficult—if it succeeds at all.

President Ronald Reagan first designated Cuba as a state sponsor of terrorism in 1982, citing Havana's support for revolutionary movements in Latin American countries such as El Salvador. The Castro regime curtailed this support in 1990. But Washington maintained sanctions anyway, ostensibly due to Cuba's "harboring of past terrorists" from foreign militant groups like Colombia's FARC. This, as William S. Leogrande noted in Foreign Affairs, is a stretch. "The case for Cuba's designation was never strong and has gotten weaker with every passing year," he wrote.

Though removing Cuba from the list would have significant symbolic value, its practical effect will be muted. Most of the financial sanctions against Cuba are delineated in the U.S. trade embargo, which remains very much in place. Independent tourism to the island, for Americans, remains forbidden. American businesses cannot invest in Cuba, and Cuban businesses cannot invest in the United States. Overturning the embargo requires discarding existing legislation, something the White House cannot do without cooperation from Congress. Republican opposition makes this unlikely.

Full restoration of ties with Cuba would also require overcoming several other legal and logistical obstacles. American citizens and corporations have filed nearly 6,000 property claims against the Castro regime, which nationalized all U.S. property on the island. Cuba, meanwhile, has demanded compensation for economic hardship wrought by the U.S. trade embargo as well as from the CIA's covert operations against the island in the 1960s.  In its secret negotiations with the Castro regime, Washington has refused to cut funding for covert programs aimed at empowering political opposition in Cuba or cease broadcasting anti-Castro television and radio networks. The U.S. provides Cuban medical personnel with incentives to defect while traveling or working in a third country, and refuses to grant visas to Cuban academics who seek to attend conferences in the country.

Even if President Obama wished to do away with these policies, Republican opposition in Congress would surely prevent him from doing so. Critics of normalization, such as Senator Marco Rubio, claim that re-establishing ties with Havana would only strengthen the political position of the Castro regime. Rubio, considered a likely presidential candidate in 2016, said in December that Congress would not lift the trade embargo against Cuba.

"I don’t care if 99 percent of people in polls disagree with my position," he said. "This is my position, and I feel passionately about it."

Fortunately for the Obama administration, there's precedent: In 1995, in a policy change that received broad bipartisan support, the U.S. normalized relations with Vietnam. In the ensuing two decades, the Southeast Asian country has become one of Washington's strongest allies in the region. Even so, the U.S. only removed an arms embargo against Vietnam last year, and full normalization will only occur when Vietnamese president Nguyen Phu Trong visits this year.

With respect to Latin America, Obama announced that Washington would take a more conciliatory stance towards its neighbors in the region.

"The days in which our agenda in this hemisphere so often presumed that the United States could meddle with impunity, those days are past," he said.

In practice, however, overcoming Washington's difficult past with Cuba is unlikely to happen quickly or easily.