The People's Republic of Burma
The People's Republic of Burma came into existence after the bloody Burmese Civil War, a remarkably complex civil war which saw Communists and foreign powers on both sides. In particular, the Communist Party of Burma had actually been split in whether to accept the aid of the People's Republic of China, with the "Red Flags" siding with Mao and the Yellow Flags siding with Prime Minister U Nu, their forces only defecting to the Maoist side after General Ne Win's coup put him in charge of the Anti-Fascist People's Freedom League government, which he used to subsequently purge of the Communist Party members. The Civil War began in March 1948 with the Red Flag Communists commencing combat operations against the U Nu government, and significantly accelerated in early 1949 when the Second Field Army of the People's Liberation Army arrived in Burma to assist them. The war reached its most violent stage in late 1950 with the British-backed coup of General Ne Win and the arrival of Commonwealth troops. The war would last until 1954, with the final defeat of the Commonwealth forces in the Battle of Meiktila. The war famously pitted former comrades against each other, as the most important officers of each side were World War II veterans - many of them had even served alongside each other in the British Burma Rifles fighting against Japanese forces. Most historians judged that the Civil War could have been avoided if not for the 1947 assassination of Aung San, the father of independent Burma.
In 1954, Burma was in a mess. Much of the Indian and Chinese trading population had fled Burma into neighboring Thailand. Entire swaths of Burma's landscape had been literally burned down in the fighting from Commonwealth napalm strikes, while many of the jungles and paddies of Burma were littered with Communist-placed land mines. Several of the ethnic minority regions were under the de facto control of ethnic armies, such as the Karen National Union that had revolted in 1949. Although the Burmese Communists first saw them as part of a British plot to split and divide Burma, the advice of both Mao Zedong and Zhou Enlai was to try very hard to accommodate their demands. Relations between the two groups were frosty until Ne Win's coup in 1950, warming only because the ethnic armies tended to loathe Ne Win far more than they did Communists.
Although most of the ethnic minorities demanded independence, an unsuccessful incursion by the KMT and a border war with India severely spooked leaders on both sides, forcing them to the table. Mao's last act before leaving Burma was to help forcing both sides to the table at the Yangon Truce, brokered by famous Burmese poet Kodaw Hmaing. Inspired by the orthodox Marxist nationalities policy of the Soviet Union, the Yangon Truce included a promise by the ethnic militias to be subsumed into the broader (Burmese) People's Army. Simply put, the leaders of Communist Burma simply feared any future border conflicts (both China and India were viewed as hostile) and were willing to make any concession to gain the military manpower of the ethnic armies. In return, most of the ethnic regions were designated as autonomous regions, similar to the Soviet Socialist Republics. With the flight of Mao but a significant PLA presence in Burma, Zhou Enlai was left around to run North Chinese operations in Burma. Although the Burmese Communists were extremely suspicious of Zhou, he expertly diffused tension by simply not really ever intervening in Burmese politics. The PLA Second Field Army, although technically an independent foreign force, more or less took all orders from the General Secretary of the Communist Party of Burma, Than Thun. It was definitely possible to sneak out Mao from Burma, as well some other major leaders, but there wasn't a really good way to extricate the PLA 2nd Field Army from Burma, so there they stayed.
At home, Than Thun proved to largely be a Maoist-inspired radical in the territory directly governed by the Communist Party of Burma. Land was redistributed to the peasants, with mass killings on the basis of class orchestrated by the new government. Ironically, Chinese (and Indians) were disproportionately likely to be landlords and merchants, and much of the Chinese population of Burma fled into China or Thailand. The participation of the People's Liberation Army in persecuting Chinese was widely displayed in China proper, but it also meant that Than Thun didn't view them as a threat. The Indian population also largely fled into India proper. Out of a population of 20 million, the Burmese Civil War killed an estimated 700,000 people, while the mass killing of landlords and other opponents of the new Communist regime was estimated to have killed up to 100,000 people. A border war almost sparked immediately between India and Burma, as the Indian public was outraged over the persecution of Indian merchants, the United Kingdom begged the Indians to keep fighting such a war, and the remnants of Ne Win's Army fled to India, setting up a government-in-exile that was largely sober and reasonable due to the lack of Ne Win. A major outcome of this was a proxy border war, where the United Kingdom would funnel equipment into India and the Pakistanis/Soviets into Burma.
Burma was only beginning to recover from both the brutal civil war and Communist repressions, when Zhou Enlai decided it was time to use up all of the political capital that he had accumulated. In late 1956, both the North Chinese divisions of the Burmese People's Army (BPA), as well as many of the more militaristic members of the Burmese People's Army (most of them), aching for a fight and rather confident in their abilities after driving British forces out of Burma, decided it was time to repay the debt to the nation that had rescued their revolution. Not to mention that Burma was technically a member of the State Union (although the least integrated one), and they were in a since obligated. Than Tun eventually agreed, largely as he saw overall sentiment in the BPA was overwhelmingly in favor. BPA forces, avoiding the most common mountain passes to China, made a famous but very costly climb over the various mountains that was quickly compared to Hannibal crossing the Alps. Arriving in Yunnan, they landed into the one province that perhaps loathed the central KMT government more than any other. They found no shortage of peasants to join their army, while the actual local government, still angry that the central KMT had persecuted and exiled their long-time leader, Long Yun, simply stood at the sidelines. The Southwest proved to be an immediate problem zone, as much of the lightly guarded rural mountainous areas in Yunnan were lost to the KMT, forcing the KMT to redirect a significant amount of troops from the offensive against the PRC down South to retake Yunnan.