Short answers, pulled from the story.
Japan invaded Thailand to secure ports, railways, and airfields needed to conquer Malaya and Burma. The Japanese military required free passage through Thai territory to execute their expansionist goals without conflict with the Thai army.
Colonel Yoshihide Hayashi commanded the Number 82 Section during its final planning stages. This group also known as the Taiwan Army Research Unit or Doro Nawa Unit consisted of thirty men formed in 1939 or 1940.
The Japanese presented Thailand with an ultimatum at 23:00 on the 7th of December 1941. Thais were given two hours to respond before military action commenced.
Thailand maintained a well-trained military force totaling 26,500 men. A reserve force brought total army numbers up to approximately 50,000 personnel.
Thai youth soldier cadets from Sriyaphai Secondary School pinned down the Japanese 1st Infantry Battalion. Regular army units and Provincial Police joined the defense until fighting ended in the afternoon.