When Thailand joined the Axis powers, the Thai government had to declare war on the Allies and was forced to use military force to support the combat operations of the Imperial Japanese Army (IJA) by moving the troops of the Royal Thai Army (RTA) to capture Kengtung to be the defense of Burma, which was a territory controlled by the British Raj.

At that time, the Royal Thai army had just finished Franco-Thai War only 7 months, the equipment of many units were in the process of being repaired.

In early 1942, Chinese forces led by Generalissimo Chiang Kai-shek invaded the former Siamese Shan States, when the Imperial Japanese Army had to invade Burma's territory to the Indian border The Imperial Japanese Army's rear line would become a weak point for British and Chinese troops to attack the Imperial Japanese Army's rear line.

During the 3rd Infantry Division's military occupation of Kengtung, soldiers had to face disease and also the problem of a shortage of supplies and clothing.

The Royal Thai Armed Forces Supreme Command Headquarters (RTARFSCH) had also ordered to establish a field police department to perform administrative duties, law enforcement, maintain order in occupied areas, and suppress the insurgency in the occupied areas.

When the Empire of Japan surrendered on 14 August 1945, the Thai government rushed to declare peace, and returning these territories to the United Kingdom, but in good faith the term "Siamese Shan States" appeared in history only for a short time.