The Ovambo and their Nkhumbi neighbours live in a flood plain, which is artificially divided by the present frontier between Angola and Namibia. From the mid-nineteenth century until World War I, they underwent a process of underdevelopment and class formation linked to the evolution of commercial relations with western societies. Between 1845 and 1885 the ivory trade temporarily enriched the Ovambo and widened the productive base of their economy through the introduction of fire-arms. At the same time, however, fire-arms became a necessity, and thus forged permanent links of dependence on western societies. Cattle replaced ivory as an export item after the elephants had been shot out, but pressure on the Ovambo's own cattle resources were largely avoided by systematic raiding in southern Angola. After the turn of the century natural disasters and effective Portuguese resistance to raiding made this solution inoperative, and led to a general impoverishment of Ovambo society. The social impact of this impoverishment was extremely uneven, for the kings and their followers passed it on to the more vulnerable members of society through a system of harsh and arbitrary taxation. A new stratum of men without cattle was thus forced to turn to migrant labour in Namibia and Angola. The colonial conquest of 1915 froze this situation into a permanent system of recurrent labour migration.