Despite the relative inaccessibility of the highlands, the Batak groups have been deeply shaped by influences from neighboring cultures. Many words for Batak political leaders and religious concepts show Indian influence, as do Batak divination and astrological lore. Border areas such as Karo and Mandailing model their traditional political systems on the nearby state societies, Aceh and Minangkabau. Islam was introduced to the southern Batak lands from Minangkabau in the 1820s, on the eve of the Dutch incursion into the area. By the 1850s, they had established a civil administration in the southern Batak areas, a region they hoped to use as a buffer between Muslim Aceh and Muslim Minangkabau. The Dutch gradually extended their control northward through Toba, encountering armed resistance from the charismatic warrior chief, Sisingamangaraja XII. By 1910 all Batak areas were under Dutch control, schools had been established in Toba, Angkola, and Mandailing, and missionary Christianity was thriving in Toba. By the 1920s, literate Batak had established a cosmopolitan city culture of newspapers and book publishing in Medan and Sibolga; writers were turning their attention to nationalist and anti-Dutch concerns. North Sumatra was occupied by the Japanese from 1942 to 1945. Since the Indonesian national revolution of 1945-1949, the region has remained an economically vital part of the Indonesian state. Owing to population pressure on over-used farmland, out-migration to cities continues.