and other American markets. By the beginning of the 19th century, Portugal controlled outposts at six locations in Africa. One was the Cape Verde Islands, located about due west of Dakar, Senegal. Claimed for Portugal by Diogo Gomes about 1458, this archipelago of eight major islands was devoted to sugar cultivation using slaves taken from the African mainland. The Portuguese once had extensive claims on the West African coast—since they were the first Europeans to explore it systematically—but by 1800 they were left with only a few ports at the mouth of the Rio Geba in what is now known as the Guinea-Bissau. To the east, the Portuguese controlled the islands of São Tomé and Príncipe, located south of the mouth of the Niger River. Like the Cape Verde Islands, they were converted to sugar production in the early 16th century using slaves acquired on the mainland in the vicinity of the Congo River. By the end of the 19th century, Portuguese landowners had successfully introduced cocoa production using forced African labour. Further south, the Portuguese claimed both sides of the mouth of the Congo River, as well as the Atlantic coast as far south as the Rio Cunene. In practical terms, Portugal controlled port cities like those of Cabinda (north of the Congo River mouth), Ambriz (south of the Congo's mouth), Luanda and Benguela (on the Angolan coast) plus some river towns in the Angolan interior. The last area claimed by Portugal in Africa was along the southeast coast on either side of the mouth of the Zambezi River. After reaching this area, known as the Swahili Coast, at the end of the 15th century, the Portuguese came to dominate most of it by the end of the 16th century. During the 17th century, they lost control of everything north of Cape Delgado to Arabs from Oman (who established the Sultanate of Zanzibar), leaving them with major ports at Mozambique, Quelimane, and Lourenço Marques, plus settlements along the Zambezi and other rivers. Despite these holdings, the Portuguese hold in Africa was problematic. The first cause was the small size of Portugal's population, coupled with the lack of popular support for overseas empire. Exploration and conquest began as an enterprise supported by the nobility, and Portuguese peasants rarely participated unless forced to do so. When the common people of Portugal did chose to emigrate, they were much more likely to