In May 1946, the border dispute between France and Thailand reached its climax. On 24 May a detachment of 800 French soldiers crossed the Mekong River from Laos (then a part of French Indochina) and, with air and artillery support, clashed with local forces in pursuit of the communist rebels. Thailand could do nothing to deter the invasion.
On 26 May, Thailand appealed to the UN Security Council, asking it to prevent the French aggression. Representatives of France argued that its soldiers only pursued Chinese and Vietnamese communist bandits who raided the area east of the river, from Siam territory.
These events wrought tremendous change in Thailand life: soon, for reasons which remain unclear, King Ananda Mahidol was shot and Bhumibol Adulyadej ascended the throne as Rama IX. There were also several military coups in the country.
However, despite the internal political struggle, Thailand, compared with the rest of the countries in the region, was an oasis of stability. The state joined the Southeast Asia Treaty Organization, taking a consistently anti-communist stance.
On 19 December 1946, France entered the bloody Indochina War, which lasted until 20 July 1954, and eventually resulted in it losing its dominions in the region completely.
Source:
Windrow, Martin. The French Indochina War 1946-1954, Osprey, 1988.