The main objective of the Japanese was to prevent China from importing arms and fuel through French Indochina along the Kunming–Haiphong railway, from the Indochinese port of Haiphong, through the capital of Hanoi to the Chinese city of Kunming in Yunnan.

[1] Although an agreement had been reached between the French and Japanese governments prior to the outbreak of fighting,[2] authorities were unable to control events on the ground for several days before the troops stood down.

Japanese leaders then dreamed of creating what they called a Greater East Asia Co-Prosperity Sphere, an economic and political coalition of Asian nations tied to Japan.

In early 1940, troops of the Imperial Japanese Army (IJA) moved to seize southern Guangxi and Longzhou County, where the eastern branch of the Kunming–Hai Phong Railway reached the border at the Friendship Pass in Pingxiang.

On 10 July, the French parliament voted full powers to Marshal Philippe Pétain, effectively abrogating the Third Republic.

On 19 June, Japan took advantage of the defeat of France and the impending armistice to present the Governor-General of Indochina, Georges Catroux, with a request, in fact an ultimatum, demanding the closure of all supply routes to China and the admission of a 40-man Japanese inspection team under General Issaku Nishihara.

[4][5] On 22 June, while Catroux was still in his post, the Japanese issued a second demand: naval basing rights at Guangzhouwan and the total closure of the Chinese border by 7 July.

Issaku Nishihara, who was to lead the "inspection team", the true purpose of which was unknown, even to the Japanese, arrived in Hanoi on 29 June.

The still neutral United States had already been contracted to provide aircraft, and there were 4,000 Tirailleurs sénégalais in Djibouti that could be shipped to Indochina in case of need.

Both governments then "instructed their military representatives in Indochina to work out the details [although] they would have been better advised to stick to Tokyo–Vichy channels a bit longer".

Decoux and Martin, acting on their own, looked for help from the American and British consuls in Hanoi, and even consulted with the Chinese government on joint defence against a Japanese attack on Indochina.

[7] On 6 September, an infantry battalion of the Japanese Twenty-Second Army based in Nanning violated the Indochinese border near the French fort at Đồng Đăng.

During the Sino-French War of 1884–1885, the French had been forced into an embarrassing retreat from Lạng Sơn in which equipment had likewise been thrown into the same river to prevent capture.

[9] Among the units taken captive at Lạng Sơn was the 2nd Battalion of the 5th Foreign Infantry Regiment that contained 179 German and Austrian volunteers, whom the Japanese in vain tried to induce to change sides.

A French Government envoy came to negotiate; in the meantime, shore defenses remained under orders to open fire on any attempted landing.

On 9 December 1940, an agreement was reached whereby French sovereignty over its army and administrative affairs was confirmed, while Japanese forces were free to fight the war against the Allies from Indochinese soil.

[11][12] In May 1971, a document from senior North Vietnamese political figure Trường Chinh was published in English translation which denounced allegations that the Japanese where treated as liberators after assisting the removal of the French, even afterwards turning on the local Viet Minh resistance forces and committing atrocities.

At that point, Vietnamese nationalists under the Viet Minh banner took control in the August Revolution, and issued a Proclamation of Independence of the Democratic Republic of Vietnam.