A 3,000-mile (5,000 km) band of semi-arid country extending across Africa south of the Sahara desert, the Sahel zone ("the shore" in Arabic) passes through Mauritania, Senegal, Mali, Burkina Faso, Niger, Chad, and the Cape Verde Islands. Similar semi-arid conditions prevail in Sudan, Ethiopia, and Somalia. These countries are among the poorest in the world. The low annual rainfall in this region is variable (4-20 in or 10-50 cm) and falls in a short, intense period in July and August. In some years the rains fail to develop, and droughts are a common occurrence. The uncertain rainfall of the Sahel makes it generally unfavorable for agriculture.

For centuries, the indigenous nomadic Tuareg people used the Sahel in a sustainable way, constantly moving herds of camels from one grazing area to the next; they practiced little agriculture. In the nineteenth and twentieth centuries, following European colonization, herds of water-dependent, non-native...