Nepal is believed to have been populated mainly by immigration over many centuries, of Mongloid groups from the Tibetan region and of Indo-Aryan groups from northern India. The Muslim invasion of India accelerated migration from the south, beginning in the thirteenth century, when a large number of Rajputs (ruling groups of north India) and other Hindus left the Gangetic plain to find refuge in the Hills of Nepal. The ruggedness of the Nepalese Hills kept them safe from any sustained or unified invasion from either the south of north. The high Himalayas formed a nearly impregnable barrier in the north, with the south being protected by dense malarial forests in the Terai and lower hills.
All accounts of Nepal's history focus on the Hill region, which was also the centre of political activity. Very little is known about the political or economic history of the Terai before unification of Nepal in 1769. Before then, most of the Terai was under the jurisdiction of various Hill states and principalities.
