Thailand's ruling house, founded by Rama I, who, under the title of Chao Phraya Chakkri (military commander of the Chao Phraya area), had played an important role in the struggle against Burma. Chakkri became king of Thailand in 1782 following the execution of his predecessor. As Rama I (q.v.), Chakkri reigned until 1809. His reign marked the reorganization of Siamese defenses to repel Burmese attacks in 1785, 1786, 1787, 1797, and 1801. His descendants reigned in an unbroken line after him.

For more than 100 years, Thai kings followed an isolationist policy toward Europeans after the so-called Phaulkon-Tachard conspiracy of 1688, but the reign of Rama II (q.v.; 1809–24) witnessed a renewal of official contacts with foreigners at the end of the Napoleonic wars. Agreements were reached with Portugal in 1818. A mission of the British East India Company visited Bangkok in 1822, followed shortly thereafter by the first British resident merchant.

The reign of Rama III (q.v.; ruled 1824–51) was marked by a limited increase of trade with European powers. A treaty was negotiated with the East India Company in 1826, followed by a similar treaty with the United States in 1833.

The strongly traditionalist concept of monarchy embodied by the first three rulers of the Chakkri dynasty did not survive under the rising tide of Western power and influence. King Mongkut (q.v.), Rama IV (reigned 1851–68), reoriented the policy of his government to accommodate that influence. He was forced to surrender a degree of Thai legal and fiscal independence, but his nation was saved from suffering Western invasion or permanent domination. His policies were continued and developed by his son King Chulalongkorn (q.v.), Rama V (reigned 1868–1910). Both monarchs endeavoured to modernize their state along Western lines with the help of European advisers. The reforms of Mongkut and Chulalongkorn, along with the need of Britain and France for a buffer state between their colonies, enabled Thailand, alone among Southeast Asian nations, to escape Western colonial rule.

The reign of King Vajiravudh (q.v.), Rama VI (reigned 1910–25), was characterized by social reforms. Although the king was somewhat isolated from his people, he negotiated a series of treaties restoring full fiscal autonomy to Thailand. A plot to restrict the king's power and impose a constitution was aborted in 1912.

King Prajadhipok (q.v.), Rama VII (reigned 1925–35), was the last of the absolute monarchs. He advocated constitutional government but failed to promote popular understanding of such a policy or enlist support from the political elite. On June 24, 1932, the so-called Promoters Revolution ended absolutism and instituted constitutionalism, though from 1933 the government was generally dominated by the military. Prajadhipok abdicated in 1935.

King Ananda Mahidol (q.v.), Rama VIII (reigned 1935–46), was allied with Japan and during World War II declared war on Great Britain and the United States. In June 1946 the king was shot and his younger brother, Bhumibol Adulyadej (q.v.), succeeded him as Rama IX (reigned 1946– ). As a constitutional monarch, Bhumibol wielded little real political power, functioning instead as the ceremonial head of state.