Sabah or ‘North Borneo’, as it was formerly known, used to be part of the domain of the Sultanate of Sulu. Sabah was ceded to the British North Borneo Company in 1878 by the Sulu Sultanate and upon independence, Sabah joined the Federation of Malaysia in 1963 and remained as a Malaysian province until today. The Sulu Sultanate was finally annexed as part of the Philippines by the American colonial government in 1915. Nevertheless, as a successor state of the Sulu Sultanate, the Philippines had never relinquished its claim over Sabah. The territorial intrusion by the Royal Army of the Sulu Sultanate in 2013 into Sabah has sparked unease amongst both Sabahans and the Malaysian government. The Sultan of Sulu claimed that Sabah has always been part of his kingdom. Is this claim valid under international law?