In the pre-colonial (Sumatra before Dutch) period, the origins of the term Batak as a form of self-ascription is elusive. In 19th century European accounts, mainly based on interviews with coastal Malays, Batak had the connotation of interior people, pork-eaters, and uncivilized cannibals. The Malay and Batak identities were once mutually exclusive, at least on the east coast of Sumatra. However, the Batak and Malay distinction was not racial but cultural. If a Batak converted to Islam, he ceased to be Batak and became Malay. Islam was perhaps the most definitive Malay marker.
In the early 19th century, Stamford Raffles proposed a policy that the Islamic lands of Acheh and Minangkabau should be kept apart by making the Batak lands a Christian block. Raffles was also the architect of the Anglo-Dutch Treaty of 1824 that irrevocably and arbitrarily divided the cultural unity of Sumatra and the peninsula. The contemporary boundary between Indonesia and Malaysia is a legacy of that treaty.
The Dutch authorities also maintained a ‘wedge policy’ – the strategy of keeping the two Islamic bulwarks of Aceh and Minangkabau separated by a belt of non-Muslims in the Bataklanden. Indeed the Dutch encouraged the Christian mission into the north, once it was clear that Mandailing were highly resistance to Christian evangelism inspite of achieving the conversion of some Mandailings in Pakantan.
The Transformation of Traditional Mandailing Leadership in Malaysia and Indonesia in the Age of Globalization and Regonal Autonomy by Abdur-Razzaq Lubis, 2001