The colonial era began in the 1859 with the French conquest of Sai Gon. In 1862 Viet King Tu Duc was forced to cede control of the south, which became the Protectorate of 'Cochinchina. Two years later King Norodom agreed to the establishment of the Protectorate of Cambodia, which was upgraded 20 years later to the status of a colony. By the late 1880s Protectorates of 'Annam' (central Viet Nam) and 'Tonkin' (north Viet Nam) had also been created. By this time the French had resolved to annex the Lao territories, believing they held deposits of precious metals and that the Mekong River offered a 'back door' into China. They were also concerned to prevent their imperial rival Britain from manipulating Siamese interests in the region and eager to pacify the mountainous north, which from the 1870s had been periodically disturbed by armed bands of renegade Black, Yellow and Striped Flag mercenaries from China.
In 1886 France signed a treaty with Siam establishing a vice consulate in Luang Prabang under Auguste Pavie, and in the following year the French staked their claim to the kingdom based on its status as a tributary of Viet Nam, which they already controlled. When Luang Prabang was sacked in 1887 by a joint force of Tai Khao and Chinese Black Flag rebels under Sip Song Chu Tai leader Kham Hum, French troops rode in and rescued King Oungkham (1868-1895), who gratefully accepted French protection for his kingdom.
In 1893, with French gunboats menacing Bangkok, Siam reluctantly ceded all of the territories east of the Mekong to France. The central and southern provinces were initially incorporated as one of the five associated regions of Indochina, while the northern kingdom of Luang Prabang remained a French protectorate, but in 1899 all of the Lao territories became a single administrative unit. Further agreements with Siam in 1904 and 1907 added parts of Sayaburi and Champassak provinces west of the Mekong to French Indochina. The province of Houaphanh was appended to Luang Prabang in 1933.
The French approach to colonial administration in Laos has been described as one of 'benign neglect'. Headed by a Résident superieur based in Vientiane, the colonial government was staffed by only a few hundred French civil servants at any given time, and while the royal court in Luang Prabang continued to manage its own affairs, the day-to-day running of the territories was entrusted largely to Vietnamese civil servants in Vientiane. Meanwhile since there was no industry to speak of and agriculture was barely self-sufficient, there was little money for infrastructural development such as roads, schools and hospitals.
Ironically it was the French themselves who unwittingly sponsored the idea of modern nationhood amongst the disparate Lao territories, primarily in an attempt to remove the Lao people from the cultural orbit of neighbouring Siam, which increasingly aspired to the creation of a 'Greater Siam' made up of all the Tai-speaking territories. During the 1930s and 1940s, through the auspices of a small French-educated Lao elite, Lao language and literature was promoted and the first Lao-language Lao history books appeared in print. During this period too, in recognition of the central educational role of Buddhism in Laos, efforts were made to reorganise and give national character to the Lao Buddhist sangha through the establishment of Buddhist Institutes in Vientiane (1929) and Luang Prabang (1932), the restoration of Ho Phra Keo and the promotion of Vientiane's Wat Sisakhet and Wat Ong Tu as centres of Buddhist ceremonial. In the early 1940s a Service de propagande Lao was set up to 'awaken the Lao national spirit', and in the so-called Samay Funefou Xat era which followed the French government launched a bi-weekly newspaper called Lao Nyai ('Great Laos'), which ran poetry competitions celebrating Lao culture and history and contained features which sought to trace the 'glorious lineage of the modern Lao' back to the kingdom of Lane Xang.
The events of World War II forever shattered the image of French supremacy, giving fresh impetus to nationalistic sentiment in Laos. Following the Japanese occupation of French Indochina in March 1945, King Sisavangvong was obliged to declare an independent state, but when the Japanese surrendered five months later he quickly moved to re-establish the French protectorate. At this juncture Prime Minister and Hereditary Uparat (Viceroy) Prince Phetsarath took over leadership of Lao Issara ('Free Laos') – a resistance movement originally formed against the Japanese – and in open defiance of the king proclaimed an independent and unified Laos in September 1945. King Sisavangvong responded by dismissing Prince Phetsarath and was subsquently placed under house arrest and forced by the National Assembly to abdicate. However, in March 1946, at the request of Lao Issara, the king re-ascended the throne as constitutional monarch of all the Lao territories.
Notwithstanding these events, the Allies had decided that the French should return, and just a few days after the king's coronation French paratroopers moved north from Champassak to Thakhek, where they defeated a joint Lao Issara-Viet Minh force led by Prince Phetsarath's half-brother Prince Souphannavong. Re-occupying Vientiane and Luang Prabang in April 1946, the French endorsed the unity of Laos as a constitutional monarchy within the French Union, and in the following year elections were held which led to the appointment of the first Royal Lao Government. However, the Viet Minh subsequently stepped up their offensives against French rule in Viet Nam, and in the years which followed the French government realised that if it was to preserve any of its former empire it would have to lighten its colonial burden. Prince Phetsarath and his half-brothers Prince Souphannavong and Prince Souvanna Phouma, who had fled to Siam in 1946 to set up a Lao Issara government-in-exile, were therefore invited to enter into formal negotiations for the granting of greater autonomy to Laos. The more moderate Prince Souvanna Phouma subsequently returned under amnesty to Vientiane where he helped draw up the Convention of 1949, which recognised Laos as an 'independent associate state' within the French Union. However, neither Prince Phetsarath nor Prince Souphannouvong were satisfied with French conditions for independence; Prince Phetsarath subsequently made his home in Thailand, while Prince Souphannouvong returned to north west Viet Nam and in alliance with the Viet Minh established a resistance movement known as the Lao Patriotic Front (Neo Lao Hak Xat), which quickly established a firm foothold in the north eastern provinces of Houaphanh and Phongsali.
In 1953 the French government granted full sovereignty to both Cambodia and Laos. Early the following year French forces suffered a calamitous defeat at Dien Bien Phu, obliging them to sue for peace at the Geneva Conference of 1954, the terms of which ended French involvement in South East Asia and effected the fateful division of neighbouring Viet Nam along the 17th parallel between the communist north and the capitalist south.