Thailand officially adopted a position of neutrality until it was invaded by Japan in December 1941. At the start of the Pacific War, the Japanese Empire pressured the Thai government to allow passage for the Japanese troops to invade British-held Malaya and Burma. The Thai government under Plaek Phibunsongkhram (known simply as Phibun) considered it preferable to co-operate with the Japanese rather than fight them. Axis-aligned Thailand declared war on the United States and Britain and proceeded to the annexation of territories in neighbouring countries, expanding northwards, southwards and eastwards, and gaining a border with China near Chiang Tung (Keng Tung).