By signing the Anglo-Siamese Treaty of 1909, Thailand relinquishes its sovereignty over the Malay states of Kedah, Kelantan, Perlis and Terengganu, which become British protectorates.
The Anglo-Siamese Treaty of 1909 or Bangkok Treaty of 1909 was a treaty between the United Kingdom and the Kingdom of Siam signed on 10 March 1909, in Bangkok. Ratifications were exchanged in London on 9 July 1909, and the treaty established the modern Malaysia–Thailand border. The area around modern Pattani, Narathiwat, southernmost Songkhla, Satun, and Yala remained under Thai control, where decades later the South Thailand insurgency would erupt. Thailand relinquished its claims to sovereignty over Kedah (Thai: ไทรบุรี, romanized: Saiburi), Kelantan (กลันตัน, Kalantan), Perlis (ปะลิส, Palit) and Terengganu (ตรังกานู, Trangkanu) which entered the British sphere of influence as protectorates. These four states, along with Johor, later became known as the Unfederated Malay States.