Critics, however, question whether, in the rush to reward progress, the U.S. has lost its leverage should Myanmar backtrack.
Walter Lohman, director of the Asia program at the conservative Heritage Foundation think tank, said the administration was right to normalize diplomatic relations but moved too quickly to suspend investment and trade sanctions.
There are unresolved ethnic conflicts, a constitution skewed in favor of the military and political prisoners still in detention. National elections in 2015 are widely viewed as key to consolidating reforms.
"We won't really know whether the U.S. going so far and so fast on sanctions was the right thing to do for at least a year or so yet," said Lohman, who recommended the U.S. set benchmarks Myanmar should meet for sanctions to be lifted entirely. "The military could still call this whole thing off if they want to."
Questions linger about whether elements within the military are acting independently of Thein Sein. Despite his order to stop fighting, Myanmar's army pressed an offensive against ethnic Kachin rebels that has displaced an estimated 70,000 people in the north.
"The army clearly wants to remain a strong force and there are probably divisions between the uniformed army and the ex-generals who run the government," Clapp said.
The senior U.S. official said Myanmar has yet to sever its military relationship with North Korea, which Thein Sein has committed to do, and the U.S. is continuing to raise the issue with the government.
Since the start of the policy of engagement with Myanmar — which reversed two decades of pressure and diplomatic isolation — a key U.S. goal has been to end North Korean weapon sales to Myanmar, which, if they are continuing, violate U.N. Security Council resolutions and could help pay for Pyongyang's nuclear and missile programs.
Yet the Obama administration appears to have decided that engaging the Myanmar military will be more productive than keeping it at arm's length. Myanmar was invited to observe U.S. military exercises in Thailand in February.
Aung Din, a U.S.-based activist and former political prisoner, views that as a seal of approval for an army still fighting its own citizens and committing atrocities. He said it would be better to get military chiefs in Indonesia and the Philippines — Southeast Asian nations that have shifted from authoritarian rule to democracy — to engage their Myanmar counterparts before the U.S. does.
He advocates more U.S. engagement with Myanmar's diverse ethnic minority groups, who have been fighting the military for decades and whose longstanding grievances need to be addressed for the country to achieve peace.
But Clapp, the former charge d'affaires, cautions there's only so much Washington can do to solve Myanmar's internal problems, including the Buddhist-Muslim unrest, beyond counseling what might be the best course of action.
"We can't get involved and stop it on the ground," she said. "It's their issue, it's their test."
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