YANGON, Myanmar >> The dictators and army generals of Myanmar have ceded political power one drop at a time over the past decade, a process they carefully managed.

Until this week.

The landslide election victory Sunday by their longtime antagonist, Aung San Suu Kyi, leader of the country’s democracy movement, stunned the military-backed governing party, members of the party and government officials said Friday.

"All of our calculations were wrong," said Zaw Htay, deputy director general of the office of the president. "It was like a tsunami."

Even as official results on Friday finally pushed Suu Kyi’s party over the threshold of a solid parliamentary majority, the great unanswered question was why the military leaders who controlled the country for 53 years would voluntarily hand over power.

The answer now seems to be that they never thought they would.

"This is payback for the last 50 years," Zaw Htay said. The governing party miscalculated voter preferences in ethnic areas, he said, and did not understand that "swing voters" would support Suu Kyi.

Having promised to respect the election results, the governing party and military leaders have to some extent painted themselves into a corner.

"This is an unexpected situation," said Hla Maung Shwe, a senior adviser to the Myanmar Peace Center, a government organization that helps lead peace talks in rebellious ethnic areas. "The government and the military would have had Plan A, B and C, but now Plan Z has just happened."

He said the governing party had counted on forging alliances with ethnic parties and winning at least 130 seats to form a majority. Instead, Suu Kyi’s party swept a great number of seats in the ethnic areas, and so far, the governing party has won just 40 seats.

"All of their pre-election expectations went wrong," Hla Maung Shwe said.

They were not the only ones who got it wrong. Suu Kyi’s party, the National League for Democracy, considerably underestimated its own strength.

Nyan Win, the party spokesman, said the National League for Democracy had expected to win around 60 percent of the vote but seemed on track to win more than 80 percent. He attributed the stunning margin to residual hatred of the military. But he also said the governing party was hurt by infighting, including the forced removal of its chairman just months before the election.

"They have no unity," he said.

As of Friday afternoon, the country’s election commission had announced the election results for 468 of the 491 parliamentary seats contested in the election. The National League for Democracy has won 378 seats compared with the governing party’s 40. Smaller parties have taken the rest.

Complete results are expected in the coming days, but Suu Kyi’s party has already won a majority, and with it the power to choose the next president.

For the past quarter-century, Myanmar’s military has been pursuing what the generals have called a road map to "discipline-flourishing democracy." In 2008, they introduced a constitution they believed was sufficiently stacked in their favor. It reserves 25 percent of the seats in parliament for the military, which meant that the military’s political wing needed only another 25 percent plus one to obtain a majority.

To seal the deal, the generals added a clause barring Suu Kyi from the presidency. She has said she will choose the president and govern by proxy. She has also said she would seek to amend the constitution, but the military has veto authority over amendments, setting up another political battle down the road.

There is still no guarantee that the military will allow her party to come to power. They canceled the results of the last election that her party won in a landslide, in 1990.

But analysts say their defeat was so public — and so transparent — that it would be more difficult this time to ignore the results.

The military establishment, which has craved recognition and foreign investment from the outside world, especially the West, repeatedly vowed that the election would meet world standards.

The International Foundation for Electoral Systems, a nonprofit organization based in Washington and funded by the United States and other Western governments, helped organize the election. About 10,000 election monitors, including teams from the European Union, Japan and the United States, blanketed the country to observe the vote.

"They put themselves in a straitjacket," said Aung Zaw, an author and the founder of a Burmese news site, The Irrawaddy. "They promised free and fair elections and promised to honor the result. And they could not back out."

Unlike in single-party states like China or Vietnam, democracy has in recent years been an agreed-upon end point for both the military and the opposition. The military seized power in a coup in 1962, but after an upheaval in 1988, it pledged to return the country to democracy. The generals made it clear it would be on their terms and at their pace.

Yet even as they followed their road map, they seemed out of step with the country’s mood. Suu Kyi was arrested in 1989, before the last election, and freed and arrested twice more before finally being freed in 2010. The 15 years she spent under house arrest, during which she won a Nobel Peace Prize, only strengthened her image as a fighter for democracy.

The generals "don’t have a good understanding of the people," said Win Min, a senior researcher at the Vahu Development Institute, a Bangkok-based organization set up by Harvard-trained Burmese exiles. "They have their own expectations that are not realistic and not reflecting the feelings of the people. They failed to gauge the mood of the people."

There has been wide speculation about a deal, whether Suu Kyi promised the military leaders something in exchange for their allowing the election to proceed unimpeded. But officials on both sides denied there was any quid pro quo.

Nyan Win, the National League for Democracy spokesman, said the party had repeatedly sought a "dialogue" with the military, but its attempts had failed until now.

Nor does the military, which retains considerable power, necessarily need a concession from Suu Kyi.

But the loss of the executive and legislative branches has nonetheless been a shock. Members of the governing party say they are perplexed.

"We had confidence in our party policies and our candidates," said Tint Zaw, a central committee member of the governing party. "But people just wanted change," he said. "They wanted a change of flavor."

© 2015 The New York Times Company