Thailand, officially the Kingdom of Thailand and historically known as Siam (the official name until 1939), is a country in Southeast Asia on the Indochinese Peninsula. With a population of almost 66 million, it spans 513,115 square kilometres (198,115 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.
Thai peoples migrated from southwestern China to mainland Southeast Asia from the 6th to 11th centuries. Indianised kingdoms such as the Mon, Khmer Empire, and Malay states ruled the region, competing with Thai states such as the Kingdoms of Ngoenyang, Sukhothai, Lan Na, and Ayutthaya, which also rivalled each other. 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 the 18th century, until it was destroyed in the Burmese–Siamese War. King Taksin the Great quickly reunified the fragmented territory and established the short-lived Thonburi Kingdom (1767–1782), of which he was the only king. He was succeeded in 1782 by Phutthayotfa Chulalok (Rama I), the first monarch of the current Chakri dynasty. Throughout the era of Western imperialism in Asia, Siam remained the only state in the region to avoid colonization by foreign powers, although it was often forced to make territorial, trade, and legal concessions in unequal treaties. The Siamese system of government was centralised and transformed into a modern unitary absolute monarchy during the 1868–1910 reign of Chulalongkorn (Rama V). In World War I, Siam sided with the Allies, a political decision made in order to amend the unequal treaties. Following a bloodless revolution in 1932, it became a constitutional monarchy and changed its official name to Thailand, becoming an ally of Japan in World War II. In the late 1950s, a military coup under Field Marshal Sarit Thanarat revived the monarchy's historically influential role in politics. During the Cold War, Thailand became a major ally of the United States and played an anti-communist role in the region as a member of SEATO, which was disbanded in 1977.
Apart from a brief period of parliamentary democracy in the mid-1970s and 1990s, Thailand has periodically alternated between democracy and military rule. Since the 2000s, the country has been in continual political conflict between supporters and opponents of twice-elected Prime Minister of Thailand Thaksin Shinawatra, which resulted in two coups (in 2006 and 2014), along with the establishment of its current constitution, a nominally democratic government after the 2019 Thai general election, and large pro-democracy protests in 2020–2021, which included unprecedented demands to reform the monarchy. Since 2019, it has been nominally a parliamentary constitutional monarchy; in practice, however, structural advantages in the constitution have ensured the military's continued influence in politics.
Thailand is a middle power in global affairs and a founding member of ASEAN. It has the second-largest economy in Southeast Asia and the 23rd-largest in the world by PPP, and it ranks 91st by nominal GDP per capita. Thailand is classified as a newly industrialised economy, with manufacturing, agriculture, and tourism as leading sectors.