The most ancient Mon-Khmer settlement of which anything definite is known was Sukhothai (located on the river Me Yom some 200 miles north of the site of modern Bangkok), which by 300 B.C. was already a sizable village. At first putting forth no pretensions to the status of kingdom, the community evidently increased rapidly in importance, for some two centuries later the chief, Phraya Thammarat, declared himself King of the district, founded the new capital of Sawankhalok, and appointed one of his sons viceroy of Sukhothai, which itself soon grew into a fortified city. Thereafter, the two towns served alternately as the capital of a country which, as the Kingdom of Sukhothai-Sawankhalok, gradually grew to great wealth and strength.

Its monarchs occupied themselves with the waging of war against the petty chieftains of neighboring states (founded in the same manner and upon the same principles as their own but at somewhat later dates) and, in course of time reducing all of them to vassalage, came to be recognized as rulers of the whole country. The vague overlordship of Cambodia continued for many centuries but with little or no influence upon the destinies of its nominal dependency, which was left to manage itself and its own subordinates as seemed to it best.

At the same time as the various Mon-Khmer states of Siam were struggling to subdue each other, the Lao tribesmen inhabiting the mountainous districts to the north, emboldened by their increasing numbers and constantly raiding the rich villages of the plains, were demanding an ever greater amount of attention and as early as the fifth century A.D., the reduction of the Lao had become almost the main preoccupation of the kings of Sukhothai-Sawankhalok. Expeditions against them were constant, but while they were frequently defeated and large numbers of them carried captive to Sukhothai or Sawankhalok, the intercourse thus brought about served only to strengthen them, since it enabled them to adopt the customs and civilization of the conquerors and then turn the acquired knowledge against their instructors with an ever-growing degree of success.

About A.D. 575, a Lao city, built in imitation of the Khmer capitals, was founded at a spot about 250 miles north of Sawankhalok and given the name of Haribunchai (later corrupted to Lamphunchai and the modern Lamphun). The chief of this town married a princess of the Khmer state of Lopburi and established a dynasty which closely followed the Brahman rites and ceremonies in vogue at Sukhothai. During this time other Lao states arose and the time soon came when the Khmer could no longer hold the Lao in check. During succeeding centuries Lao armies advanced far south into the Mon-Khmer kingdoms, marital and political alliances between Lao and Khmer royalty became common, and Lao settlements were established in various parts of southern Siam.

Despite wars with rival states to the south and the Lao to the north, the Kingdom of Sukhothai-Sawankhalok prospered greatly and in time attained to a high civilization. The arts were encouraged, the people were well governed, trade was extensive, and friendly relations were maintained with China and other distant countries by frequent exchange of embassies. Envoys from the Emperor of China, who visited Sukhothai in the seventh century A.D., have left records which indicate that the populace were chiefly engaged in the cultivation of rice and the manufacture of sugar and that in manners and customs they closely resembled the modern inhabitants of Siam. The style of architecture, remains of which still survive, followed, in somewhat degenerated form, that seen in the ruins of Angkor and other Cambodian cities.

During the reign of the hero-King Rama Khamheng (Phra Ruang) the country reached the zenith of its greatness and when he died, about A.D. 1090, he left to his heir an empire which embraced much of the Lao states to the north and all of the more southern Khmer kingdoms of Siam. This heritage, however, was fated to endure but a short time. During the eleventh century the Khmer King of Lopburi and the Lao King of Lamphun, both vassals of Phra Ruang, had been intermittently at war with each other without interference from the suzerain; toward the end of the century Lopburi was finally overcome and, declaring itself subordinate to Lamphun, was forced to admit large numbers of Lao to settle within its borders. Soon after Phra Ruang's death, a great Lao army composed of the warriors of several allied states and led by a chief known as Suthammarat, invaded Sukhothai-Sawankhalok itself, defeated its armies, overran its lands to the south, reduced the cities, and founded the capital of Pitsanulok, southwest of Sukhothai and in the heart of the Khmer Kingdom. Thereafter, although the rulers of Sukhothai-Sawankhalok continued for some time to maintain regal state, they were never again to hold a paramount position and were, in fact, to become mere vassals of the ancient enemy until eventually, some four centuries subsequent to the foundation of Pitsanulok, they were to be no more than provincial governors representative of the kings of Ayuthia.

Suthammarat, an admirer of the Khmer, in setting up his throne in the conquered kingdom, imitated as closely as possible the ways of Sukhothai and, by marrying a lady of the country, set an example for his following which gave great impetus to that fusion of Lao and Khmer which, already begun in Lopburi, was soon to result in the evolution of the Thai (Siamese) race.

The early thirteenth century saw the beginning of the last and greatest influx of Lao into the south of Siam. The suppression of the Lao-Tai undertaken in southwestern China, culminating in the decisive victories of the Emperor Kublai Khan, drove many thousands of these people down into the mountainous regions of northern Siam, where the newcomers upset the balance of power among their predecessors and caused the disruption of several of their states. As a result, many impoverished petty chieftains of ancient lineage gathered their people together and set off down the rivers to seek new fortunes in the kingdoms to the south. During the following century, mingling with the Khmer and the Lao-Khmer and acquiring great strength of numbers, the Lao wrested control from the original inhabitants and established capitals of their own, one of which, Supanburi, was in time to become dominant over all the rest. When, at the middle of the fourteenth century, Phra Chao Uthong, King of Supanburi, fleeing from a pestilence, marched westward to found a new capital, Nong Sano, now the seat of the weak successors of the great Suthammarat, fell into his hands almost without a struggle, its King fled to Cambodia, and Uthong erected near the fallen city the new city of Maha Nakhon Si Ayuthaya (Ayuthia), which was destined to become famous throughout the world as the capital of one of the greatest kingdoms in the history of Farther India.