Following the division of Lane Xang into the three rival kingdoms of Luang Prabang, Vientiane and Champassak, Siamese influence in the region grew steadily, and by the mid 18th century Ayutthaya was exacting tribute from all three. Notwithstanding the destruction of Ayutthaya by Burmese troops in 1767 (part of a campaign in which Luang Prabang was occupied for seven years by the Burmese), the Siamese quickly regrouped under General Taksin and reasserted their authority over the three territories. The sacred pha bang, which had been moved to Vientiane in 1707 following the breakup of Lane Xang, was captured by the Siamese and carried off to Bangkok in 1779, although it was returned a few years later when Nanthasaen (1781-1795) was installed as king of Vientiane by Siamese King Rama I, who believed that it had brought bad luck to Siam.
Nanthasaen was followed on the throne of Vientiane in 1795 by his younger brother Inthavong (Sai Setthathirat III), and subsequently in 1805 by his youngest brother Anou (Sai Setthathirat IV), who is regarded by the Lao people as a national hero for his valiant but ultimately disastrous rebellion against Siamese overlordship. Following Bangkok's agreement in 1820 to install Anou's son as king of Champassak, King Anou resolved to throw off the Siamese yoke, annex Luang Prabang and re-establish the kingdom of Lane Xang. Strengthening his ties with Viet King Minh Mang, Anou mounted a three-pronged invasion of Siam in 1827, marching on Bangkok with armies from Vientiane, Roi Et and Ubon under the pretext that he was coming to help Siam resist a British invasion. As soon as the Siamese realised his true intentions, they assembled a large army to drive King Anou back to the Mekong River, where his troops suffered a resounding defeat. Following the fleeing Lao troops across the river, the Siamese captured and laid waste to Vientiane in 1828, incorporating it into their territory and forcibly resettling many thousands of its residents in Siam. In the subsequent conflict between Siam and Viet Nam over territory in the north east, these were joined by thousands more from Muang Phuan (Xieng Khouang), as a result of which there are today more Lao-speaking people in north eastern Thailand (Isaan) than there are in Laos itself. Once again, the sacred pha bang was removed to Bangkok, remaining there until 1867 when it was finally returned to Luang Prabang. King Anou himself fled to Hua but was later captured and brought in a cage to Bangkok, where he died in 1835, bringing the Vientiane monarchy to an end.
Meanwhile in the south, following the death of King Pham Ma Noi (1813-1819), Champassak became a province of Siam. From 1826 until 1893 it was administered on behalf of Bangkok by a succession of governors drawn from the former Champassak royal family. Consequently on the eve of the French colonial period the only distinct sovereign entity that remained of the former kingdom of Lane Xang was Luang Prabang, the rulers of which found themselves increasingly torn between the rival powers of Siam and Viet Nam.