The prehistory of the Durand Line is complicated, involving new forms of tribal al-legiance-building, tribal and localised state-building and, finally, regional empire-building. A new tribal as well as mercantile religion and civilisation, Islam, gave rise to new patterns of trade and a new crossroads and commercial centre: Kalat. With the decline of the Mughals a Kalat state based on a local dynasty and two tribal fed-erations emerged as a semi-independent state. The British Empire ultimately strength-ened and enlarged, balanced and controlled this tribal and regional state. Thanks to the British the Khan of Kalat controlled most of present-day Baluchistan. From the 1870s the construction of this imperial state served new geostrategic designs: the Khan of Kalat conceded control over the Bolan Pass as well as Quetta to the British Empire. It was this imperial bridgehead, called British Baluchistan, which served as the cornerstone for a forward border policy and, ultimately, the Durand Line.