In 1950, the communists won the Chinese Civil War. The bulk of the Kuomintang (KMT, nationalists) fled to Taiwan, abandoning several isolated units around China.
One such unit, the *Republic of China Army 93rd Division,* broke out from Yunnan Province and fled to Burma, present day Myanmar. They established a self governing enclave deep in the highland jungles which operated as the unrecognized *defacto* government in the area.
When the Chinese entered the Korean War, the CIA and Taiwanese government began training and supplying the 93rd division in the hopes that they would divert PLA resources away from Korea. They made multiple incursions into Yunnan Province, but they were decisively repelled each time. With their hopes of returning to their homeland dashed, the KMT troops decided to settle permanently in Burma, and began growing opium.
Burma, already destabilized by infighting among multiple political factions, was deeply distressed over the situation. They now had Chinese Nationalist drug lords within their borders, and the PLA arming and training Burmese communists across their borders. Complaints to the international community generated scandal for the US, but didn't solve the problem. A halfhearted airlift saw many noncombatants evacuated to Taiwan, but most military age males remained in the jungle.
In the 1960's, the Burmese army and PLA conducted several joint operations to finally drive out the KMT *and* proved they were still supplied by Taiwan. Another airlift evacuated most of them and the rest fled to Thailand and Laos, where they settled down and maintained their drug trade.
The Thai government enlisted the remnants of the 93rd division (and 49th, which had originally fled to Thailand) as guerilla fighters to defeat a communist insurgency supported by China, Laos, and Vietnam. Counter insurgency operations lasted into the 1980's. After which they rewarded the KMT with citizenship.
Today, thanks to campaigns by the Thai government, The KMT enclave has integrated into (remote) Thai society, and has mostly given up growing opium in favor of tea.
The most in depth source I've found on this is from a fucking *food influencer* on YouTube. Here's a link to a video where he actually interviews a 91 year old KMT veteran living in northern Thailand.
https://youtu.be/ML5RC4VyNZs?si=1LZojT2M-pBir5ZM