Thailand officially adopted a neutral position during World War II 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. After the invasion, Thailand capitulated. The Thai government under Plaek Phibunsongkhram 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 which had been lost to France – as an ally against Western imperialism. Following added pressure from the start of the Allied bombings of Bangkok due to the alliance with Japan, 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.

Thailand, officially the Kingdom of Thailand and historically known as Siam, is a country in Southeast Asia on the Indochinese Peninsula. With a population of almost 70 million, it spans 513,120 square kilometres (198,120 sq mi). Thailand is bordered to the northwest by Myanmar, to the northeast and east by Laos, to the southeast by Cambodia, to the south by the Gulf of Thailand and Malaysia, and to the southwest by the Andaman Sea; it also shares maritime borders with Vietnam to the southeast and Indonesia and India to the southwest. Bangkok is the state capital and largest city.