Can you elaborate on this?
In 1909 Siam ceded the four northern Malay states (Perlis, Kelantan, Trengganu, and Kedah) to the British. These the Japanese handed back in 1943.
The army's 18th Mixed (there was a battalion of marines) Brigade was the main occupational force; the last military governor of Siamese Malaya was Lt. Gen. Luang Phairirayordet.
Here's the explanation Tojo gave at the Imperial Conference of May 31, 1943:
In policy toward Thailand we are upholding its dignity as an independent state and giving guidance to induce positive accord on cooperation in the war. Due to the enemy strategy of seizing upon the people's lack of freedom due to the pro-Japanese policy of the Phibun regime, the presence of the Japanese army, and the activities of anti-government bad elements, feelings toward Japan are certainly not satisfactory. Because of the troubled position of the Pibun regime and the psychological tendencies of the people, based upon the Japanese-Thai alliance, we will return lost territories in Malaya from the area we have occupied.