To prevent further Iban migration upriver in the Rajang River basin, which was creating conflicts with the Orang Ulu, Rajah Charles Brooke built Baleh Fort at Nanga Balleh, the confluence of the Rejang and Baleh rivers between Kanowit and Song in late 1874.
During that time, the Ibans were also engaged in conflict with the Orang Ulu of northern Sarawak, the Bidayuh of southern Sarawak, the Kantu and other Indonesian ethnic tribes from eastern Sarawak; the Iban successfully controlled the western coastline of Sarawak.