In the north, the incessant fighting amongst the Hausa city-states and the decline of the Bornu Empire gave rise to the Fulani people gaining headway into the region. Until this point, the Fulani a nomadic ethnic group primarily traversed the semi-desert Sahelian region, north of Sudan, with cattle and avoided trade and intermingling with the Sudanic peoples. At the beginning of the 19th century, Usman dan Fodio led a successful jihad against the Hausa Kingdoms founding the centralised Sokoto Caliphate. The empire with Arabic as its official language grew rapidly under his rule and that of his descendants, who sent out invading armies in every direction. The vast landlocked empire connected the east with the western Sudan region and made inroads down south conquering parts of the Oyo Empire (modern-day Kwara), and advanced towards the Yoruba heartland of Ibadan, to reach the Atlantic Ocean. The territory controlled by the empire included much of modern-day northern and central Nigeria. The sultan sent out emirs to establish a suzerainty over the conquered territories and promote Islamic civilization, the emirs in turn became increasingly rich and powerful through trade and slavery. By the 1890s, the largest slave population in the world, about two million, was concentrated in the territories of the Sokoto Caliphate. The use of slave labour was extensive, especially in agriculture. By the time of its break-up in 1903 into various European colonies, the Sokoto Caliphate was one of the largest pre-colonial African states.