Thailand known formerly as Siam and officially as the Kingdom of Thailand, is a country in Southeast Asia. It is located at the center of the Indochinese Peninsula, spanning 513,120 square kilometers (198,120 sq mi), with a population of almost 70 million people. Thailand is bordered to the north by Myanmar and Laos, to the east by Laos and Cambodia, to the south by the Gulf of Thailand and Malaysia, and to the west by the Andaman Sea and Myanmar. It also shares maritime borders with Vietnam in the Gulf of Thailand to the southeast, and Indonesia and India (Andaman and Nicobar Islands) on the Andaman Sea to the southwest. Bangkok is the nation's capital and largest city. Nominally, Thailand is a constitutional monarchy and parliamentary democracy; however, in recent history, its government has experienced multiple coups and periods of military dictatorships.

Tai peoples migrated from southwestern China to mainland Southeast Asia from the 11th century; the oldest known mention of their presence in the region by the exonym Siamese dates to the 12th century. Various Indianized kingdoms such as the Mon kingdoms, Khmer Empire and Malay states ruled the region, competing with Thai states such as the Kingdoms of Ngoenyang, Sukhothai, Lan Na and Ayutthaya, which rivalled each other. Documented European contact began in 1511 with a Portuguese diplomatic mission to Ayutthaya, which became a regional power by the end of the 15th century. Ayutthaya reached its peak during cosmopolitan Narai's reign, gradually declining thereafter until being ultimately destroyed in the 1767 Burmese–Siamese War. Taksin quickly reunified the fragmented territory and established the short-lived Thonburi Kingdom.