The far east of what is today Yemen historically formed the Sultanate of Mahra, which became part of Britain's Aden Protectorate in 1866, though retaining a degree of autonomy.[2] In this period Oman was under an informal British protectorate. Oman had asserted its rule over the south-western Dhofar region in 1829.[3] A border between the Aden and Oman was not demarcated during the colonial period, though British government documents suggest that the Ras Darbat Ali cape had been fixed as Oman's westernmost point as early as 1933–35, with a British government document recording that "it had been agreed that the question was not at that time an important one."[4] A British letter of 1947 states that "the Frontier is shown as being on the west side of Ras Dharbar Ai in a Survey of India map of south-east Arabia 1:2,000,000, but on what authority this was done is not known. The mountain is locally recognised as being the boundary between Dhofar country and Mahra country on the coast at least, though to what extent this applied inland is not clear."[5]