Thailand in World War II officially adopted a position of neutrality until the five hour-long Japanese invasion of Thailand on 8 December 1941 which led to an armistice and military alliance treaty between Thailand and the Japanese Empire in mid-December 1941. At the start of the Pacific War, the Japanese Empire pressured the Thai government to allow the passage of Japanese troops to invade British-held Malaya and Burma. The Thai government under Plaek Phibunsongkhram (known simply as Phibun) considered it profitable to co-operate with the Japanese war efforts, since Thailand saw Japan – who promised to help Thailand regain some of the Indochinese territories (in today's Laos, Cambodia, and Myanmar) which had been lost to France – as an ally against Western imperialism. Axis-aligned Thailand declared war on the United Kingdom and the United States and annexed territories in neighbouring countries, expanding to the north, south, and east, gaining a border with China near Kengtung.