Towards the end of World War II, a Japanese coup d'état in French Indochina, known as Meigo Sakusen (Operation Mei-go), took place on 9 March 1945. French Indochina comprised the colony of Cochinchina and the protectorates of Annam, Cambodia and Tonkin, and the mixed region of Laos. The French Indochinese government had remained loyal to the Vichy regime after the fall of France in June 1940.
After the coup, the Japanese replaced French officials and military personnel in the colonial infrastructures, but otherwise left them more or less intact. They did urge declarations of independence from the traditional rulers of the different regions, creating a new Empire of Vietnam, Kingdom of Cambodia and Kingdom of Laos under their direction. A small-scale campaign of guerrilla warfare followed, in which Free French Forces and their native allies on the one hand and the nationalist Viet Minh and Indochinese Communist Party on the other both fought the Japanese. In August, the regime of puppet states collapsed with the surrender of Japan. Chinese, British and French forces came to fill the vacuum, but the Vietnamese Revolution under the Viet Minh, begun in August, continued.
In 1945, the Japanese feared an Allied offensive in French Indochina. The Vichy regime had ceased to exist in Europe, but its colonial administration was still in place in Indochina, though the governor Admiral Jean Decoux had recognized and contacted the Provisional Government of the French Republic
On the eve of the Japanese coup the French garrison of Indochina comprised about 65,000 men, of whom 48,500 were locally recruited tirailleurs under French officers. The remainder were French regulars of the Colonial Army plus three battalions of the French Foreign Legion. Since the fall of France in May - June 1940 no replacements or supplies had been received from outside Indochina. In August 1940 Admiral Decoux had signed an agreement under which Japanese forces were permitted to occupy bases across Indochina. At the beginning of 1945 approximately 30,000 Japanese troops were located in the country, a force that was substantially increased by reinforcements brought in from Burma in the following months.
In early March, Japanese forces were redeployed around many of the main French garrison towns, and on 9 March 1945, the Japanese delivered an ultimatum for the French troops to disarm, without warning. Those who refused were usually massacred. In Saigon, senior Japanese officers invited the French commanders to a banquet. The officers who attended were arrested and almost all were killed. In Saigon the two senior Vichy officials, General Emile-René Lemonnier and Resident Camille Auphalle, were executed by decapitation, after refusing to sign surrender documents. The most determined French resistance was at Dong Dang where a company of Tonkinese Rifles and a battery of colonial artillery held out for three days before being massacred.
The French upcountry garrisons fared better, however, and, under the leadership of Major-General Marcel Alessandri, a column of 5,700 French troops, including many Foreign Legionnaires fought its way through to Nationalist China.
The French administration was effectively dismantled. The Japanese pressed the Empire of Vietnam, the Kingdom of Laos and the Kingdom of Cambodia to declare their independence. Emperor Bảo Đại complied in Vietnam and collaborated with the Japanese. King Norodom Sihanouk also obeyed, but the Japanese did not trust the francophile monarch.
Nationalist leader Son Ngoc Thanh, who had been exiled in Japan and was considered a more trustworthy ally than Sihanouk, returned to Cambodia and became Minister of foreign affairs in May, then became Prime Minister in August. In Laos however, King Sisavang Vong, who favoured French rule, refused to declare independence, finding himself at odds with his Prime Minister, Prince Phetsarath Rattanavongsa.
The Republic of China, which had given shelter to escaped French troops, and the United States, were reluctant to start a large-scale operation to restore French authority, as they did not favour colonial rule. Both countries ordered that their forces provide no assistance to the French, but General Claire Lee Chennault went against orders, and aircraft from his 51st Fighter Group and 27th Troop Carrier Squadron flew support missions for the French forces retreating into China.
Commandos from the British liaison organisation Force 136 had been conducting minor operations in French Indochina since late 1944. After the coup, French and British reinforcements were parachuted into Indochina and conducted guerrilla operations against the Japanese. French troops who had escaped from the Japanese coup in March joined the French and British commandos to take part in the fighting. French resistance groups had more latitude for action in Laos, as the Japanese had less control over this part of the territory. However, the commandos lacked precise orders from their governments and the practical means to mount any large-scale operations.
The French and Lao guerrilla groups also lacked significant firepower, but nevertheless managed to gain control of several rural areas. In northern Vietnam, Ho Chi Minh's Viet Minh started their own guerrilla against the Japanese and established their bases in the countryside without meeting much resistance from the occupying forces, who were mostly present in the cities. The Viet Minh lacked the military force to launch a full-scale attack against the Japanese, and their actions were limited to a few attacks against IJA military posts.
In France, recently liberated from National Socialist occupation, the French Far East Expeditionary Corps was established and prepared to be sent to Indochina to fight the Japanese.
However, Japanese troops surrendered when Emperor Hirohito announced Japan's capitulation in August. On August 16, the Japanese garrisons officially handed control to Bảo Đại in the North and the United Party in the South. This, however, allowed nationalist groups to take over public buildings in most of the major cities. The Viet Minh were thus presented with a power vacuum, and on the 19th the August Revolution commenced in which the Viet Minh took power. On 25 August 1945, Bảo Đại was forced to abdicate in favour of Ho and the Viet Minh. As Bảo Đại abdicated, the Viet Minh took control of Hanoi. In most of French Indochina, the Japanese did not oppose the Viet Minh's takeover as they were reluctant to let the French retake control of the French colony. The French troops who had been arrested in March were kept in jails by the Japanese. Imperial Japanese Army troops briefly clashed with Viet Minh guerillas in Annam, but there were no major military engagements. One exception was the Thái Nguyên Province, where Japanese troops refused to surrender and did battle with the Viet Minh from 20 to 25 August 1945. They finally surrendered on 26 August 1945, and the Viet Minh could take possession of their weapons. Hô Chi Minh proclaimed Vietnam's independence on 2 September 1945.
Troops from the United Kingdom—the 20th Indian Division under Major General Douglas Gracey—and the Republic of China (National Revolutionary Army) entered the country, and started disarming the Japanese troops. They were joined by the French Far East Expeditionary Corps, which arrived in September. Jacques Massu's troops took control of Saigon.
French Indochina was left in chaos by the Japanese occupation. Admiral Jean Decoux, who had supported the Vichy regime instead of Free France, was sent to France to face trial. In Laos, Phetsarath Rattanavongsa's Lao Issara deposed the King in October and declared the country's independence, but its government had to flee in April 1946, as the French troops advanced towards Vientiane. In Cambodia, Son Ngoc Thanh was arrested by the French. Ho Chi Minh found himself in partial control of north Vietnam, setting the stage for the First Indochina War.