The was a short undeclared military confrontation between Japan and Vichy France in northern French Indochina. Fighting lasted from 22 to 26 September 1940; the same time as the Battle of South Guangxi in the Sino-Japanese War, which was the main objective as to why Japan occupied Vietnam during this time.
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.
Although an agreement had been reached between the French and Japanese governments prior to the outbreak of fighting, authorities were unable to control events on the ground for several days before the troops stood down. As per the prior agreement, Japan was allowed to occupy Tonkin in northern Indochina, and thus effectively blockade China.
Background
The occupation of Indochina also fit into Japan's long-term plans. 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. Together, these Asian countries would expel Western imperialists and capitalists then share trade, resources and commodities between themselves.
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. They also tried to move west to cut the rail line to Kunming. The railway from Indochina was the Chinese government's last secure overland link to the outside world.
On 10 May 1940, Germany invaded France. On 22 June, France signed an armistice with Germany (in effect from 25 June). On 10 July, the French parliament voted full powers to Marshal Philippe Pétain, effectively abrogating the Third Republic. Although much of metropolitan France came under German occupation, the French colonies remained under the direction of Pétain's government at Vichy. Resistance to Pétain and the armistice began even before it was signed, with Charles de Gaulle's appeal of 18 June. As a result, a de facto government-in-exile in opposition to Pétain, called Free France, was formed in London.
Franco-Japanese negotiations
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. The Americans became aware of the true nature of the Japanese "request" through intelligence intercepts since the Japanese had informed their German allies. Catroux initially responded by warning the Japanese that their unspecified "other measures" would be a breach of sovereignty. He was reluctant to acquiesce to the Japanese, but with his intelligence reporting that Japanese army and navy units were moving into threatening positions, the French government was not prepared for a protracted defense of the colony. Therefore, Catroux complied with the Japanese ultimatum on 20 June. Before the end of June the last train carrying munitions crossed the border bound for Kunming. Following this humiliation, Catroux was immediately replaced as governor-general by Admiral Jean Decoux. He did not return to France, however, but to London.
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. On 3 July, he issued a third demand: air bases and the right to transit combat troops through Indochina. These new demands were referred to the government in France.
The incoming governor, Decoux, who arrived in Indochina in July, urged the government to reject the demands. Although he believed that Indochina could not defend itself against a Japanese invasion, Decoux believed it was strong enough to dissuade Japan from invading. At Vichy, General Jules-Antoine Bührer, chief of the Colonial General Staff, counselled resistance. 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. In Indochina, Decoux had under his command 32,000 regulars, plus 17,000 auxiliaries, although they were all ill-equipped.