By means of this annexation, Axis-aligned Thailand expanded northwards to the 22nd parallel north and gained a border with China.

[3] The territory of the Northern Thai province was mountainous, except for a few small areas, such as the intermontane basin of Kengtung.

There were few roads connecting the districts and most of the population lived in small mountain villages.

Thai Prime Minister Plaek Phibunsongkhram signed a secret agreement with the Japanese Empire on 14 December 1941 and committed the Thai armed forces to participate in the planned Malayan Campaign and Burma Campaign.

As a reward for entering into a military alliance with them, the Japanese agreed to return to Thailand Kedah, Perlis, Kelantan, and Terengganu, the four Malayan provinces ceded to the British in 1909, as well as parts of Shan State in British Burma that were deemed "lost territories" of Thailand.

The defense of the Shan States had been left to the Nationalist Chinese forces, upon the request of the British.

Following the existing agreement between Thai Prime Minister Plaek Phibunsongkhram (Phibun) and the Japanese Empire, on 18 August 1943, the Japanese government agreed to the Thai annexation of Kengtung and part of Mongpan State (as well as the annexation of Kelantan, Trengganu, Kedah, Perlis states and nearby islands in Malaya.

Made up mostly of small rural communities, during the occupation the Thai territory in Shan State remained a largely forgotten place.