Generals from Myanmar's junta held peace talks in June near the border with China with representatives of three powerful ethnic armies.
They sat across a wide table covered with blue cloth and decorated with elaborate bouquets. But the rebels were playing a double-game.
Secretly, the ethnic armies — collectively called the Three Brotherhood Alliance — had laid the groundwork for Operation 1027, a major offensive launched in October.
"We were already preparing for the operation when we met them," said Kyaw Naing, a spokesman for the Myanmar National Democratic Alliance Army (MNDAA), a largely ethnic-Chinese group that is part of the rebel coalition.
Reuters interviewed a dozen resistance officials with knowledge of the operation, as well as analysts and other people familiar with the matter.
They disclosed previously unreported elements of the planning, including details of the formation of a unified battlefield brigade and the extent of China's impatience with the junta, which some analysts believe emboldened the militias.
Operation 1027 has delivered nationwide victories for the alliance and other groups fighting the military, which unseated Nobel laureate Aung San Suu Kyi's civilian-led government in February 2021.
The junta cracked down on protests after the coup, sparking a grassroots rebellion and reigniting conflict with some ethnic armies. The army says tough measures are required to fight groups it considers "terrorists".
Two members of the Three Brotherhood Alliance, together with five other armed groups, formed the new Brigade 611 in early 2022, four rebel officials said.
It was a display of unprecedented cooperation among outfits that come from different parts of Myanmar, speak different languages and traditionally have had different priorities, according to a November report from the US Institute of Peace, a Washington-based think-tank.
The operation came amid rising anger in Beijing with the junta over rampant crime on the border, which created conditions that supported the blitzkrieg, two analysts said.
As of October, more than 20,000 people, mainly Chinese, were being held in over 100 compounds in northern Myanmar, where the workers — many of them trafficked — defraud strangers over the Internet, according to a USIP estimate.
Chinese officials delivered an ultimatum in Beijing this September to their Myanmar counterparts: eliminate the compounds or China would do so, according to a person briefed on their meeting.
Operation 1027 began in northern Shan State, abutting the border with China, where troops led by the Three Brotherhood Alliance — which comprises MNDAA, the Ta'ang National Liberation Army and the Arakan Army (AA) — said they captured around 150 military outposts, five towns and four border gates within a month. Among the rebel forces was the multi-ethnic Brigade 611, said MNDAA's Kyaw Naing.
The formation includes troops from entities supported by the parallel civilian government as well as fighters from the AA, one of Myanmar's most powerful ethnic armed forces, and the Bamar People's Liberation Army, a newer militia drawn mostly from the country's majority Bamar people, rebel officials said.
The Karenni Nationalities Defence Force now controls parts of the frontier with Thailand and also contributed to Brigade 611.
Despite the setbacks, the Myanmar military — one of the largest in Southeast Asia — has sizeable resources and a "determination to prevail at all costs", said Richard Horsey, a senior adviser at the non-profit International Crisis Group.
Anti-junta operations have rapidly expanded to other parts of Myanmar, with battles in the central region of Sagaing as well as in states near India and Bangladesh.
In several areas, rebel groups are supported by the People's Defence Forces (PDF), backed by the civilian National Unity Government (NUG) that includes representatives of Suu Kyi's administration.
The NUG claims control over parts of the country and has worked on diplomatically isolating the junta. Suu Kyi remains in detention in Naypyidaw.
In Mandalay, the gateway to the northern territories, the local PDF is tasked with stalling military reinforcements to the frontline, its spokesman said.
The NUG supports over 300 PDF units under its command using money raised by taxation, bond sales and other methods.
* Reuters staff contributed to this article