French IndochinaVietnam's independence was gradually eroded by France in a series of military conquests between 1859 and 1885. In 1862, the southern third of the country became the French colony of Cochinchina.
By 1887, the entire country formally became part of French Indochina along with Cambodia and Laos. Vietnam itself was divided into three regions: Tonkin in the north, Annam in the middle and Cochinchina in the south. The French administration imposed significant political and cultural changes on Vietnamese society. A Western-style system of modern education was developed, and Roman Catholicism was propagated widely.
Developing a plantation economy to promote the export of tobacco, indigo, tea and coffee, the French largely ignored increasing calls for Vietnamese self-government and civil rights. A nationalist political movement soon emerged, with leaders such as Phan Bội Châu, Phan Châu Trinh, Emperor Duy Tân, Cường Để, Hồ Chí Minh, Ngô Đình Diệm and Nhất Linh fighting or calling for independence. However, the 1930 Yên Bái mutiny of the Vietnamese Nationalist Party (Vietnamese: Việt Nam Quốc Dân Đảng; VNQDD) was suppressed easily by the colonial government.
World War II
After the Fall of France, China launched an invasion to French Indochina on November 1, 1940. One month later, Thailand launched separate offensive, resulted to the French-Thai War. Joint invasions of the Chinese from the north and the Thais from the south driven the French out of Southeast Asia on February 4, 1941. Following the takeover of power, hundreds of VNQDD members returned from their exile in China. Vũ Hồng Khanh, a Vietnamese general served in the Chinese army, was appointed as the interim head of government of Tonkin by the Chinese military administration between February to June 1941.
On May 29, 1941, Khanh convened a national conference that was attended by representatives from Annam, Tonkin and Cochinchina in Hanoi, including Ngô Đình Diệm and Trần Trọng Kim. During this conference, Khanh promoted a unification between three regions into one national state. This notion was adopted on June 3, 1941 and the provisional government was set up on June 10, 1941. On June 19, 1941, the Republic of Vietnam (Vietnamese: Việt Nam Dân Quốc) was declared in Da Lat with Khanh was elected President of the Republic.
Shortly afterward, China offered a protectorate treaty over Vietnam as well as a military alliance. Under this condition, Vietnam, although a de facto independent state, was controlled by China politically, economically and militarily. Vietnam also established military alliance with Thailand, Cambodia and Laos, created the so-called "Da Lat-Vientiane-Phnom Penh-Bangkok Axis". This alliance actively participated during the invasions of northern Malaya in 1941 and of Burma in 1942. In 1944, the combination of economic mismanagement and natural disasters resulted to a famine in northern Vietnam, in which more than one million people died of starvation.
Internal resistance was formed during this period of famine. Vietnamese nationalists in Japan, led by Prince Cường Để, was contacted by Ngô Đình Diệm in late 1944 to form a secret group that will attempted to overthrow the Republican government and instated Cường Để as the new head of state. On other hand, General Charles de Gaulle of the Free French Forces had considered to restore Nguyễn Vĩnh San, former Emperor Duy Tân, who joined the Free French as the Emperor. Vĩnh San, who had proven his amity with France, was still a popular nationalist hero, making him an acceptable candidate for future Vietnamese head of state.