One of the oldest continuous Malay monarchies, traditionally dated to the 1136 conversion of its ruler to Islam. Kedah controlled the fertile northwestern plain of the peninsula, exported rice and tin, and spent centuries balancing tribute to Siam against Achehnese and Bugis pressure before becoming a British protectorate in 1909.
Non-terminal: British protectorate from 1909, unfederated Malay state, and since 1957 a constituent monarchy of modern Malaysia.
Standard scholarly survey covering Kedah's tributary position between Siam and Aceh (verified Open Library /works/OL3901035W).