Nang Klao died in 1851 and was succeeded by his forty-seven- year-old half brother, Mongkut (Rama IV, 1851-68). Mongkut's father, Loet La (Rama II, 1809-24), had placed him in a Buddhist monastery in 1824 to prevent a bloody succession struggle between factions loyal to Mongkut and those supporting Nang Klao (although Nang Klao was older than Mongkut, his mother was a concubine, whereas Mongkut's mother was a royal queen). As a Buddhist monk, Mongkut won distinction as an authority on the Pali Buddhist scriptures and became head of a reformed order of the Siamese sangha. Thai Buddhism had become heavily overlain with superstitions through the centuries, and Mongkut attempted to purge the religion of these accretions and restore to it the spirit of Buddha's original teachings (see Religion , ch. 2).

Mongkut's twenty-seven years as a Buddhist monk not only made him a religious figure of some consequence but also exposed him to a wide array of foreign influences. Blessed with an inquiring mind and great curiosity about the outside world, he cultivated contacts with French Roman Catholic and United States Protestant missionaries. He studied Western languages (Latin and English), science, and mathematics. His lengthy conversations with the missionaries gave him a broad perspective that greatly influenced his policies when he became king in 1851. He was more knowledgeable of, and at ease with, Western ways than any previous Thai monarch.

Mongkut was convinced that his realm must have full relations with the Western countries in order to survive as an independent nation and avoid the humiliations China and Burma had suffered in wars with Britain. Against the advice of his court, he abolished the old royal trade monopoly in commodities and in 1855 signed the Treaty of Friendship and Commerce with Britain. (This treaty, commonly known as the Bowring Treaty, was signed on Britain's behalf by Sir John Bowring, governor of Hong Kong.) Under the terms of the treaty, British merchants were permitted to buy and sell in Siam without intermediaries, a consulate was established, and British subjects were granted extraterritorial rights. Similar treaties were negotiated the next year with the United States and France, and over the next fifteen years with a number of other European countries. These agreements not only provided for free trade but also limited the Siamese government's authority to tax foreign enterprises. The elimination of these barriers led to an enormous increase in commerce with the West. This expansion of trade in turn revolutionized the Thai economy and connected it to the world monetary system.

The demand for extraterritorial privileges also convinced the king that unless Siam's legal and administrative systems were reformed, the country would never be treated as an equal by the Western powers. Although little in the way of substantive modernization was accomplished during his reign, Mongkut eliminated some of the ancient mystique of the monarch's divinity by allowing commoners to gaze on his face, published a royal gazette of the country's laws, and hired a number of Western experts as consultants, teachers, and technicians. Long-standing institutions such as slavery remained basically untouched, however, and the political system continued to be dominated by the great families. Conservatives at court remained strong, and the king's death from malaria in 1868 postponed pending reform projects.

Data as of September 1987