The traditional court histories of Lan Xang begin in the Year of the Nāga 1316 with the birth of Fa Ngum.^[15] Fa Ngum's Grandfather Souvanna Khampong was king of Muang Sua and his father Chao Fa Ngiao was the crown prince. As a youth Fa Ngum was sent to the Khmer Empire to live as a son of King Jayavarman IX, where he was given princess Keo Kang Ya. In 1343 King Souvanna Khampong died, and a succession dispute for Muang Sua took place.^[16] In 1349 Fa Ngum was granted an army known as the "Ten Thousand" to take the crown. At the time the Khmer Empire was in decline (possibly from an outbreak of the Black Death and the combined influx of Tai peoples),^[16] both Lanna and Sukhothai had been established in what had been Khmer territory, and the Siamese were growing in the area of the Chao Phraya River which would become the Ayutthaya Kingdom.^[17] The opportunity for the Khmer was to create a friendly buffer state in an area they could no longer effectively control with only a moderately sized military force.

Fa Ngum's campaign started in southern Laos, taking the towns and cities in the region around Champasak and moving northward through Thakek and Kham Muang along the middle Mekong. From his position on the middle Mekong, Fa Ngum sought assistance and supply from Vientiane in attacking Muang Sua, which they refused. However, Prince Nho of Muang Phuan (Muang Phoueune) offered assistance and vassalage to Fa Ngum for assistance in a succession dispute of his own and help in securing Muang Phuan from Đại Việt. Fa Ngum agreed and quickly moved his army to take Muang Phuan and then on to take Xam Neua and several smaller towns of Đại Việt.^[18]

The Vietnamese kingdom of Đại Việt, concerned with their rival Champa to the south sought a clearly defined border with the growing power of Fa Ngum. The result was to use the Annamite Range as both a cultural and territorial barrier between the two kingdoms. Continuing his conquests Fa Ngum turned toward the Sip Song Chau Tai along the Red and Black River valleys, which were heavily populated with Lao. Having secured a sizable force of Lao from each territory under his domain Fa Ngum moved down the Nam Ou to take Muang Sua. Despite three attacks the King of Muang Sua, who was Fa Ngum's uncle, was unable to deter the size of Fa Ngum's army and committed suicide rather than be taken alive.^[18]

In 1353 Fa Ngum was crowned,^[19] and named his Kingdom Lan Xang Hom Khao "The Land of a Million Elephants and the White Parasol", Fa Ngum continued his conquests to secure the areas around the Mekong by moving to take Sipsong Panna (modern Xishuangbanna Dai Autonomous Prefecture) and began moving south to the borders of Lanna along the Mekong. King Phayu of Lanna raised an army which Fa Ngum overwhelmed at Chiang Saen, forcing Lanna to cede some its territory and provide valuable gifts in exchange for mutual recognition. Having secured his immediate borders Fa Ngum returned to Muang Sua. ^[18] By 1357 Fa Ngum had established the mandala for the Kingdom of Lan Xang which extended from the borders of the Sipsong Panna with China^[20] south to Sambor below the Mekong rapids at Khong Island, and from the Vietnamese border along the Annamite Range to the western escarpment of the Khorat Plateau.^[21] It was thus one of the largest kingdoms in Southeast Asia.