It was a moment President Obama has sought since his inauguration, when he pledged to reach out to despotic rulers who “are willing to unclench your fist.”

In the three years since, his invitation has been ignored by Iran, North Korea and Syria.

Then came Thursday, when the leader of one of the most isolated and repressive regimes in the world — a government responsible for killing thousands in a quest to silence dissent — welcomed U.S. Secretary of State Hillary Rodham Clinton.

Burma has lurched unexpectedly toward reform in the past three months. Its move has coincided with a broader attempt by Obama to pivot his administration’s foreign policy focus away from the Middle East, to counter-balance of China’s rise in Asia.

The result was the first U.S. secretary of state ever to set foot in the gaudy presidential palace in this constructed-from-scratch capital. And Clinton carried to her meeting with Burmese President Thein Sein a letter from Obama that conveyed praise, but also a warning that significantly more progress is needed for change to take root.

“For decades, the choices of this country’s leaders kept it apart from the global economy and the community of nations,” Clinton said after the meeting. “While the measures already taken may be unprecedented and welcomed, they are just a beginning.”

The Burmese overture — and the U.S. response — are freighted with risk for both sides. Thein Sein must balance the desires of his country’s restless opposition movement against those of the hard-liners within his own government. Obama could face criticism for coddling autocrats if Burma’s reform push proves less than genuine.

Ultimately, Burma’s leaders are thought to be seeking an end to U.S. sanctions, but Clinton on Thursday offered Thein Sein much smaller incentives. The two discussed loosening restrictions on United Nations funding for health and micro-finance projects, as well as the possibility of additional international aid.

The most direct result of their meeting could be a restoration of U.S. diplomatic relations, which would upgrade the American mission here to a full-fledged embassy headed by an ambassador, Clinton told reporters afterward.

Eager to convince U.S. officials of his sincerity, Thein Sein gave a detailed a 45-minute presentation on his plans for reforming the very areas of his authoritarian government that the United States and others have criticized for decades, according to a senior State Department official who was present at the meeting.

So far, the gestures of reform have included the release of political prisoners, greater media freedoms, and plans for political and economic reform. And the Burmese leader vowed Thursday to work toward releasing more prisoners, brokering a cease-fire with ethnic minorities and adopting international agreements on nuclear weapons development.

The Obama administration has proceeded cautiously with Burma’s reclusive leaders, careful not to declare success too early in a country that has shown promises of reform before, only to resume its crackdowns.

“It was good policy by the Obama administration that led to where we are now, but they also to a degree got lucky,” said Ernie Bower, a Southeast Asia expert at the Center for Strategic and International Studies.

Burma’s decision to open up this time is considered to be at least in part an attempt to ward off an increasingly assertive China. But the reforms are also thought to be the result of internal power dynamics among Burmese leaders that U.S. officials readily admit they are still trying to understand.

Thursday’s meeting gave U.S. officials their most extensive look yet at the man thought to be driving Burma’s tentative reform efforts. Like all leaders in Burma’s nominally civilian government, Thein Sein is a retired general. But beside that, not much else is known.

He has heart disease and uses a pacemaker. He is described as low-key and served for years as prime minister, a position that was several steps below Senior Gen. Than Shwe, the junta’s longtime leader.

It is that role, U.S. officials believe, that may have planted the seed for reform.

“He spent an enormous amount of time traveling outside the country in meetings, interacting with others,” said the senior State Department official. “So it’s entirely possible that he had a chance to get a much better sense of what was going on in Southeast Asia, how far behind his country was falling.”

There are pervasive rumors in Burma, also known as Myanmar, that the president is still partly controlled by his predecessor Than Shwe, who has, at least officially, retired from public life.

“It’s just common sense,” said one Burmese journalist, speaking on the condition of anonymity for fear of retribution. “Why would he give up that power? It can only hurt him in the future. His family still has crony companies to run. He would not want another leader to come in and criticize or get rid of him.”

In Thursday's meeting, U.S. officials said they were surprised by Thein Sein’s frankness about the pressure he’s facing. He acknowledged that his reforms have drawn supporters, opponents and fence-sitters-- even within key members of his own government.

One of the main reasons Clinton was willing to visit Thein Sein was that the president’s reforms have been endorsed by Aung San Suu Kyi, the leader of Burma’s long-persecuted democracy movement. Suu Kyi has said that she believes the president’s intentions are good, even if she doesn’t trust others in his government.

On Thursday, as Suu Kyi and Clinton had dinner together — the first meeting between two of the world’s most prominent female politicians — Suu Kyi mentioned that she had been reading books of late about military personalities, said a U.S. official who was also at the dinner.

She was trying to understand the “military mentality” of commanders such as Dwight D. Eisenhower and Otto von Bismarck, who later went into politics — men much like the former general she is now trying to work with to change her country.