Short answers, pulled from the story.
Japan moved troops into southern Guangxi and Longzhou County in early 1940. This action aimed to seize the eastern branch of the Kunming, Hai Phong Railway at the Friendship Pass in Pingxiang.
Japanese leaders dreamed of creating a Greater East Asia Co-Prosperity Sphere as an economic and political coalition tied to Japan. Preventing China from importing arms and fuel through French territory became the main objective for Tokyo.
Governor-General Georges Catroux received the ultimatum demanding the closure of all supply routes to China on the 19th of June. Admiral Jean Decoux replaced Catroux immediately following this humiliation before the end of June.
Imperial General Headquarters reduced the number of required troops to 6,000 on the 21st of September. Seven and a half hours before the ultimatum expired, Martin and Nishihara signed an agreement authorizing stationing of these 6,000 troops in Tonkin north of the Red River.
Lieutenant-General Aketo Nakamura sent columns across the border near Đồng Đăng at precisely 22:00 on the 23rd of September 1940 despite the signed agreement. An exchange of fire quickly spread to other border posts overnight leading to the Battle of Lạng Sơn.