Regional instability, environmental stresses, foreign interference and global economic crises in the second and third decades of the 21st century led to the collapse of ASEAN and a regional arms race.
Thailand became the first South East Asian casualty in this increasingly tense and dangerous period. Around this time, Thailand was embroiled in chronic internal infighting between rival political factions as well as separatist movements. Thailand was so affected by various natural disasters, including the destruction to Bangkok from Typhoon May. In the 2030s a nationalist faction was elected into government and began pursuing a belligerent foreign policy to revive national pride. However, the country was in no shape for war and when yet another skirmish erupted along the Thailand-Myanmar border, Myanmar launched a full-scale invasion of Thailand.
Laos, Cambodia and Malaysia soon joined the war against Thailand, forming the infamously short-lived "Yangon Pact". While the Thai Army was initially successful in pushing back Myanman forces in the west, the entry of Laos, Cambodia and Malaysia caught the Thais off guard. The effect of the years of disunity and poor government was beginning to show. Soon much of northern and eastern Thailand and all of southern Thailand was lost.
In the First Partition of Thailand, the humiliating Treaty of Chantaburi awarded northern and western Thailand to Myanmar, parts of eastern Thailand (including ethnic Khmer areas) to Cambodia, while the Isan provinces to Laos. Only a rump Thai state remained centered on the Chao Praya valley. Malaysia refused to sign the treaty but annexed parts of southern Thailand anyway, which became the states of Pattani and Singora. The rest of Malaysian-occupied Southern Thailand declared autonomy from Bangkok. A group of warlords based at Phitsanulok also set up their own regime, autonomous in all but name. To add salt to the injury, Vietnam demanded, and received, Chonburi and Rayong (including the airbase at U Tapao), much to the annoyance of the Yangon Pact.