The Thai are related linguistically to Tai groups originating in southern China. Migrations from southern China to Southeast Asia may have occurred in the 6th and 7th centuries. Malay, Mon, and Khmer civilizations flourished in the region prior to the arrival of the ethnic Tai.

Thais date the founding of their nation to the 13th century. According to tradition, in 1238, Thai chieftains overthrew their Khmer overlords at Sukhothai and established a Thai kingdom. After its decline, a new Thai kingdom emerged in 1350 on the Chao Praya River. At the same time, there was an equally important Tai kingdom of Lanna, centered in Chiang Mai, which rivaled Sukothai and Ayutthaya for centuries, and which defines northern Thai identity to this day.

The first ruler of the Kingdom of Ayutthaya, King Rama Thibodi, made two important contributions to Thai history: the establishment and promotion of Theravada Buddhism as the official religion--to differentiate his kingdom from the neighboring Hindu kingdom of Angkor--and the compilation of the Dharmashastra, a legal code based on Hindu sources and traditional Thai custom. The Dharmashastra remained a tool of Thai law until late in the 19th century. Beginning with the Portuguese in the 16th century, Ayutthaya had some contact with the West, but until the 1800s, its relations with neighboring kingdoms and principalities, as well as with China, were of primary importance.

After more than 400 years of power, in 1767, the Kingdom of Ayutthaya was brought down by invading Burmese armies and its capital burned. After a single-reign capital established at Thonburi by Taksin, a new capital city was founded in 1782, across the Chao Phraya at the site of present-day Bangkok, by the founder of the Chakri dynasty. The first Chakri king was crowned Rama I. Rama's heirs became increasingly concerned with the threat of European colonialism after British victories in neighboring Burma in 1826.

The first Thai recognition of Western power in the region was the Treaty of Amity and Commerce with the United Kingdom in 1826. In 1833, the United States began diplomatic exchanges with Siam, as Thailand was called until 1938. However, it was during the later reigns of Rama IV (or King Mongkut, 1851-68), and his son Rama V (King Chulalongkorn (1868-1910), that Thailand established firm rapprochement with Western powers. The Thais believe that the diplomatic skills of these monarchs, combined with the modernizing reforms of the Thai Government, made Siam the only country in South and Southeast Asia to avoid European colonization.

In 1932, a bloodless coup transformed the Government of Thailand from an absolute to a constitutional monarchy. King Prajadhipok (Rama VII) initially accepted this change but later surrendered the kingship to his 10-year-old nephew. Upon his abdication, King Prajadhipok said that the obligation of a ruler was to reign for the good of the whole people, not for a select few. Although nominally a constitutional monarchy since 1932, Thailand was ruled by a series of military governments interspersed with brief periods of democracy. Since the 1992 elections, Thailand has been a functioning democracy with constitutional changes of government. On February 6, 2005, Prime Minister Thaksin Shinawatraís Thai Rak Thai (TRT) party won 377 of 500 seats in the Lower House of Parliament, and on March 9, 2005, Prime Minister Thaksin was chosen to serve a second term.

As with the rest of Southeast Asia, Thailand was occupied by the Japanese during the Second World War. Since Japan's defeat in 1945, Thailand has had very close relations with the United States. Threatened by communist revolutions in neighboring countries such as Vietnam, Cambodia, and Laos, Thailand actively sought U.S. assistance to contain communist expansion in the region. Recently, Thailand also has been an active member in multilateral organizations like the Association of South East Asian Nations (ASEAN) and the Asia-Pacific Economic Cooperation (APEC) forum.