Historical sources as well as local oral tradition suggest that settlement of Karen people in Thung Yai did not occur before the second half of the 18th century. At that time, due to political and religious persecution in Burma, predominantly Pwo-Karen from the hinterlands of Moulmein and Tavoy migrated into the area northeast of the Three Pagodas Pass, where they received formal settlement rights from the Siamese Governor of Kanchanaburi. Sometime between 1827 and 1839 the Siamese King Rama III established this area as a principality ("mueang") and the Karen leader who governed the principality received the Siamese title of nobility "Phra Si Suwannakhiri". During the second half of the 19th century, this Karen principality at the Burmese border became particularly important for the Siamese King Rama V (Chulalongkorn) in his negotiations with the British colonial power in Burma regarding the demarcation of their western border with Siam.