The increased presence of European rivals along the East coast led Arab traders to concentrate on the overland slave caravan routes across the Sahara from the Sahel to North Africa. The German explorer Gustav Nachtigal reported seeing slave caravans departing from Kukawa in Bornu bound for Tripoli and Egypt in 1870. The slave trade represented the major source of revenue for the state of Bornu as late as 1898. The eastern regions of the Central African Republic have never recovered demographically from the impact of 19th-century raids from the Sudan and still have a population density of less than 1 person/km2. During the 1870s, European initiatives against the slave trade caused an economic crisis in northern Sudan, precipitating the rise of Mahdist forces. Mahdi's victory created an Islamic state, one that quickly reinstituted slavery.