Japanese invasion of French Indochina
In September 1940, the Japanese invaded French Indochina (仏印進駐 Futsu-in shinchū ) in order to prevent the Republic of China from importing arms and fuel through French Indochina along the Sino-Vietnamese Railway, from the port of Haiphong through Hanoi to Kunming in Yunnan. The fighting, which lasted several days before the French authorities reached an agreement with the Japanese, took place in the context of the ongoing Sino-Japanese War and World War II. Japan was able to occupy northern Indochina, tighten the blockade of China and make a continuation of the drawn-out Battle of South Guangxi unnecessary.
In early 1940, troops of the Imperial Japanese Army (IJA) moved to seize Longzhou in south Guangxi, where the eastern branch of the railroad from Hanoi reaches the border, and also tried to move west to cut the rail line to Kunming. Chinese resistance, supplied from Indochina, was tough.
Then on 22 June 1940, France signed an armistice with Germany, Japan's Axis ally. This established the neutral but pro-Axis Vichy France government in the unoccupied part of France. Vichy France also controlled most French overseas possessions, including Indochina.
The IJA captured Longzhou, closing one route, but the rail line to Yunnan was still open. Japanese aerial bombing did not close it.
Japan pressured the Vichy government to close the railway, but the French did not agree.
On 5 September the South China Front Army of the IJA organized the amphibious Indochina Expeditionary Army to move into Indochina. Led by Major-General Takuma Nishimura, it was supported by a flotilla of ships, and planes from aircraft carriers and air bases on Hainan Island.
Faced with this invasion threat, Vichy France yielded. On 22 September, Japan and Vichy Indochina signed an accord which granted Japan the rights to station troops in Indochina, and to move troops and supplies through Indochina. The accord allowed up to 6,000 Japanese troops to be stationed in Indochina, with no more than 25,000 troops stationed or in transit at any given time. In addition, all Japanese land, air, and naval forces were barred from Indochinese territory except as authorized in the accord.
Fighting breaks out 
Within a few hours, columns from the IJA 5th Division under Lieutenant-General Akihito Nakamura moved over the border at three places and closed in on the railhead at Lang Son, near Longzhou. This contravened the new agreement. In the Battle of Lang Son, a brigade of French Indochinese colonial troops and Foreign Legionaires opposed the IJA until 25 September. The Japanese victory opened the way to Hanoi. Still the Vichy French had defenders in the north and south, and fresh battalions in position on the route from Lang Son to Hanoi.
On 23 September, Vichy France protested the breach of the agreements by the IJA to the Japanese government.
On the morning of 24 September, Japanese aircraft from aircraft carriers in the Gulf of Tonkin attacked French positions on the coast. A Vichy envoy came to negotiate; in the meantime, shore defenses remained under orders to open fire on any attempted landing.
On 26 September, Japanese forces came ashore at Dong Tac, south of Haiphong, and moved on the port. A second landing put tanks ashore, and Japanese planes bombed Haiphong, causing some casualties. By early afternoon the Japanese force of some 4,500 troops and a dozen tanks was outside Haiphong.
By the evening of 26 September fighting had died down. Japan took possession of Gia Lam Airbase outside Hanoi, the rail marshalling yard on the Yunnan border at Lao Cai, and Phu Lang Thuong on the railway from Hanoi to Lang Son, and stationed 900 troops in the port of Haiphong and 600 more in Hanoi. Japanese forces remained in Indochina until the end of World War II.
See also 
- Hsu Long-hsuen, and Chang Ming-kai, History of The Sino-Japanese War (1937–1945) 2nd Ed., 1971. Translated by Wen Ha-hsiung, Chung Wu Publishing; 33, 140th Lane, Tung-hwa Street, Taipei, Taiwan, Republic of China. Pg. 317
- Conflict in Indochina, French newsreels archives (Les Actualités Mondiales), January 15, 1941
- L'Indochine française pendant la Seconde Guerre mondiale, Jean-Philippe Liardet
- Prisoners of the Japanese
- (French) French Indochina during World War II (dossier), Dr. Jean-Philippe Liardet
- "Occupation of French Indochina", Nippon News, No. 18. in the official website of NHK.