Satun and Perlis had been part of the Malay Sultanate of Kedah, but only Satun (with a mixed Thai and Malay population) remained with Thailand. Patani, Narathiwat, southermost Songkhla and Yala were historically parts of the Malay Sultanate of Patani, which had long been tributary to the Thais.
The British reason for sanctioning the continued Thai rule of the remaining northern half of the Malay provinces was the perceived value of Thailand as a friendly buffer against the French in Indochina.
Both signatories of the 1909 treaty had previously agreed to the Burney Treaty in 1826. The Burney Treaty stated that Kedah, Kelantan, Perlis and Terengganu were Thai provinces while Penang and Province Wellesley belonged to the British while Thailand would not interfere with British trade in Kelantan and Terengganu.
This agreement has had a long lasting effect on both Thailand and the Federation of Malaysia. The border between them was mainly drawn by this treaty.
The incremental tide of discontent generated by the Anglo-Siamese Treaty of 1909 may have, in part laid the foundations for the South Thailand insurgency in Pattani, Yala and Narathiwat from 2004 to the present.