Before Lan Xang the middle Mekong was a landscape of muang, small principalities held together by personal allegiance and tribute rather than by any single crown, and oriented variously toward Angkor, toward the Tai polities to the west, and toward Dai Viet. Fa Ngum's achievement was to weld them into one kingdom and to give that kingdom a religious ideology it had not previously possessed as a unified system.
His route to power ran through exile. His father was expelled from Muang Sua and the family found refuge at the Khmer court, where Fa Ngum was raised, educated and, the chronicles say, married to a Khmer princess. He returned in the middle of the fourteenth century with Khmer military backing, took the muang of the Mekong in sequence, and was consecrated at Muang Sua in 1353. The consecration is the point at which a Tai chieftaincy becomes a Buddhist kingship on the Khmer and Mon model, with the ruler positioned as protector of the religion and the religion in turn conferring legitimacy on the ruler.
The arrangement did not save him personally. He was deposed by his own officials around 1372, reportedly for conduct they judged intolerable, and died in exile at Nan. His son Samsenethai, whose regnal name records a census of three hundred thousand Tai, consolidated what his father had assembled. The Buddhist establishment Fa Ngum introduced outlasted both of them by six centuries, and the sangha remained tied to the Lao state through every subsequent change of regime, including the socialist one that abolished the monarchy in 1975.