Short answers, pulled from the story.
Japan invaded French Indochina primarily to cut off China's last secure overland supply route, the Kunming-Haiphong railway, which ran from the port of Haiphong through Hanoi to Kunming in Yunnan. Severing this corridor prevented China from importing arms and fuel during the Sino-Japanese War. The occupation also fit Japan's broader aim of building a Greater East Asia Co-Prosperity Sphere.
Fighting lasted from the 22nd to the 26th of September 1940. An agreement had been signed between French and Japanese military representatives seven and a half hours before the Japanese ultimatum expired, but Japanese ground forces crossed the border anyway at 22:00 on the 22nd of September.
The French position at the railhead at Lang Son was surrounded by Japanese armour and forced to surrender on the 25th of September 1940. Before surrendering, French commanders ordered the breechblocks of the 155mm cannons thrown into a nearby river to prevent the Japanese from using them. Among the prisoners taken was the 2nd Battalion of the 5th Foreign Infantry Regiment, which included 179 German and Austrian volunteers who refused Japanese attempts to get them to change sides.
A December 1940 agreement confirmed French administrative sovereignty while granting Japan the right to station troops and fight the Allies from Indochinese soil. On the 28th of July 1941, some 140,000 Japanese troops entered southern French Indochina. Japan deposed French authorities entirely in the spring of 1945, and Vietnamese nationalists under the Viet Minh then declared independence, leading to the First Indochina War in 1946.
On the French side, Governor-General Georges Catroux accepted Japan's initial ultimatum before being replaced by Admiral Jean Decoux, and Maurice Martin served as supreme commander of Indochinese troops during formal negotiations. General Issaku Nishihara led the Japanese delegation and issued the ultimatum that set the September 22nd deadline. Major-General Takuma Nishimura commanded the amphibious Indochina Expeditionary Army.
Vietnamese views were divided. Some saw Japan's humiliation of a Western colonial power as evidence that European rule could be overthrown. However, occupation was also experienced as harsh, contributing to the Vietnamese famine of 1945. A 1971 document attributed to North Vietnamese leader Truong Chinh stated that after neutralizing French resistance, Japanese forces attacked Viet Minh base areas, burning homes and committing atrocities, and that all Japanese inducements for the Viet Minh to cooperate were rejected.