By Andres Oppenheimer

While President Obama's announcement Wednesday that he will normalize relations with Cuba is the biggest diplomatic breakthrough with the island after six decades of hostilities, his speech may have been less "historic" than he portrayed it, according to numerous congressional sources and Cuba experts.

"I don't think this is going to amount to much," said Jaime Suchlicki, head of the Cuban Institute at the University of Miami. "Regardless of what President Obama said today, he needs congressional approval for making any major changes in U.S. relations with Cuba."

Suchlicki and other experts cited four reasons the announcement in the short term might not turn out to be as significant as many in the administration believe.

First, while the 1960 U.S. embargo on Cuba has gradually eroded over the decades, and the United States has become the largest supplier of food and agricultural products to the island, U.S. economic sanctions remain in place and can only be lifted by Congress. Overall, American companies are still barred from buying from or selling to Cuba, or extending credit to it without government permits.

In his speech, Obama announced, among other things, plans to open a U.S. embassy in Havana, an increase in exceptions to the ban on travel to Cuba, an expansion of commercial activities, and possible U.S. support for loans from multilateral financial institutions to Cuba. Obama, who also announced the release of U.S. subcontractor Alan Gross from a Cuban prison, said the measures are "the most significant changes in our [Cuba] policy in more than 50 years."

Second, with the Republican takeover of Congress, a lifting of the embargo is unlikely. There will be strong pressure from Cuban American legislators to block Obama's measures, the experts say.

Sen. Marco Rubio (R., Fla.) is already denouncing Obama for allegedly exceeding his presidential authority with the announcements of increased U.S. travel and commerce to Cuba. Sen. Bob Menendez (D., N.J.) said that when Congress convenes in January, he will urge Bob Corker (R., Tenn.), incoming chairman of the Senate Foreign Relations Committee, "to hold hearings on this dramatic and mistaken change of policy."

Third, while there has been no U.S. embassy in Cuba since 1961, the United States has had a permanent diplomatic mission on the island ever since. It is already one of the largest foreign diplomatic missions on the island, and its transformation into an embassy may only be a change in name.

"This is a game of smoke and mirrors," said Frank Calzon, head of the Center for a Free Cuba. "Many people ignore the fact that there are already more American diplomats in Cuba than there are Canadian, Spanish, or even Russian diplomats there."

Sen. Lindsay Graham (R., S.C.), who is scheduled to become chairman of the subcommittee that handles State Department issues, posted on Twitter Wednesday: "I will do all in my power to block the use of funds to open an embassy in Cuba."

Fourth, some Cuba watchers argue that the Castro regime may sabotage the normalization talks because it needs to maintain a confrontation with the United States. Cuba has long argued that it cannot allow fundamental freedoms on the island because it is under attack from the United States, and a full normalization of U.S.-Cuban ties would undermine its main argument to maintain a police state, they say.

In his address announcing his agreement with Obama to swap prisoners and start normalization talks, Cuban leader Raul Castro appeared in full military uniform and stressed that the U.S.-Cuban confrontation is far from over.

"This does not mean that the main [thing] has been resolved," Castro said. "The economic, commercial, and financial blockade that causes enormous human and economic damages to our people must end."

In the past, Cuba often sabotaged U.S. efforts to improve bilateral relations, historians say.

After President Jimmy Carter extended an olive branch to Cuba in the late 1970s, Castro unleashed the 1980 Mariel boatlift that resulted in a major crisis for the Carter administration. When Bill Clinton tried to improve ties, the Castro regime in 1996 shot down two Miami-based Brothers to the Rescue planes that were helping Cuban refugees lost at sea.

But Raul Castro may not react as his older brother did. In addition, Cuba is facing a possible loss of Venezuela's oil subsidies, and Raul Castro may be most interested in improving ties with Washington to jump-start the island's economy.

"[U.S.] trade and financial liberalization bodes well for the Castro regime, which is facing economic strain as its largest benefactor, Venezuela, is facing a liquidity crisis and has been forced to reduce its aid to Cuba," says Risa Grais-Targow, a Latin American analyst with the Eurasia Group. "The government had few options but to accelerate the pace of Cuba's opening, given its lack of access to international markets and financial institutions."

Andres Oppenheimer is a Latin America correspondent for the Miami Herald. aoppenheimer@miamiherald.com