MALAYAN UNDER SIAM.

Malayan States formerly under Thai/Siamese sovereignty

Webster's New Geographical Dictionary, 1988, states:

Perlis was until 1821 subject to Kedah; made separate state by Siamese 1841; came under British protection by treaty of 1909, in which Siam ceded to Great Britain its right over the state.

In Asian Frontiers, Alastair Lamb, 1968, pp. 170-171:

In the first half of the 19th century the Bangkok dynasty considered the entire Malay peninsula to fall within its sphere of influence. In the north this was real enough. (...) In 1943 the Japanese undid the territorial transfer of 1909 and restored Kelantan, Trengganu, Perlis and Kedah to nominal Thai sovereignty. (...) Japan's defeat in 1945 automatically brought the 4 northern Malay states back under British rule. It is not difficult to find Thais who will speak with regret of these lost Malayan territories.

Jarig Bakker, 13 Nov 1999
Basically these four northern states used to pay annual tribute to Siam, and then Great Britain decided Malaya was properly a British sphere of influence, having held Penang since 1786 and Province Wellesley since 1790 from Kedah for the princely sum of $10,000 (still paid each year by the Federal Government to the Kedah Government), and had to protect its commercial interests in its hinterland in the Malay states. The 1909 treaty of Bangkok allowed Great Britain to have Resident Advisors in the northern states, i.e. for them to become British protectorates.
[As for the "regret," I believe] the Thais have enough problems with their southern Muslim minorities who would be more happy in Malaysia.
Andrew Yong, 13 Nov 1999