While campaigning in 2008, Barack Obama said it was time to come up with a new US policy towards Cuba. Now, with two years left in his second term in the White House, he has finally acted.

The president has broken with a half century of seeking to isolate Cuba by announcing plans to restore diplomatic relations with the communist-run island after a year and a half of secret negotiations.

In effect, it is a political bombshell.

As he did recently on global warming or immigration reform, Obama launched an unexpected initiative to fulfil an old promise on which even some of his most fervent supporters had just given up.

Paradoxically, the stinging Democratic loss in November's legislative elections has marked a turning point for Obama.

He has decided to ignore the criticism of his political enemies, who seem to have been caught off guard and left behind.

Keenly aware that time if running out for him to etch out a legacy, Obama is shaking off the reputation he had earned as a leader who wavers and is reluctant to make tough decisions.

At times taking on the optimistic tone of his first presidential campaign -- albeit now with more gray hair -- he is speaking out forcefully to defend his decisions.

He defended an unprecedented climate change accord with China saying he could not wait forever to act on global warming.

Then he defended a reform that will benefit millions of immigrants without residency papers in the face of what he called an unfair system and said generations of immigrants have made the United States what it is today.

On Cuba, he explained his bold move by saying he wanted to explore new avenues after a half-century-long trade embargo that has utterly failed to dislodge the communist regime in Havana.

"I do not believe we can keep doing the same thing for over five decades and expect a different result," Obama said Wednesday.

"Neither the American nor the Cuban people are well served by a rigid policy that's rooted in events that took place before most of us were born," he added.

- Potent symbol -

Jeb Bush, the former Florida governor who has thrown his hat into the ring for the Republican presidential nomination with an eye to winning the White House in 2016, led the charge against Obama.

He reiterated Republican charges that Obama is acting like a king or an emperor.

Attacking a decision that Bush said would benefit "the heinous Castro brothers," the brother of former president George W. Bush said Obama had exceeded the limits of his power.

Against that backdrop, things look tough for Obama, who says he wants to work with Congress on ending what he called the world's oldest trade embargo.

But he could be helped by a new dynamic among the American people on this issue.

Florida, a key electoral battleground, was once seen as packed with Cuban exiles fiercely opposed to any gestures towards the government in their homeland.

But a poll by Florida International University has found that two thirds of the Cuban-American population of Miami favors establishing diplomatic relations with the communist regime.

The coming months will be crucial for yet another international issue on which negotiations were launched in the utmost secrecy: the nuclear stand-off with Iran.

A provisional agreement between the major powers of the so-called "5+1" group and Iran was struck in November 2013.

A new deadline for a final accord on restricting Iran's nuclear program has been set for July 2015.

Although Washington and Tehran, sworn enemies since 1979, are far from normalizing their relations, an agreement would be an undisputed diplomatic triumph for Obama after 30 years of poisoned ties that have even led to threats of armed conflict.

In January 2009, after Obama was sworn in to this first term.

Cuban President Raul Castro wished him good luck and said he seemed like a good man. The following years have been marked by signs of detente but also moments of tension.

But now, something that was unthinkable as the week began -- a meeting between the two presidents in Cuba -- is openly raised by the White House as a possibility.

It would be a potent symbol -- a trip marking the end of one of the last chapters of the Cold War. And it would be a highlight of the Obama legacy.