 Spain regained a base for the African slave trade it relinquished in the 1479 Treaty of Alcáçovas, with the acquisition of the Portuguese islands of Annobón and Bioko or Fernão Pó, plus the mainland between the Niger River and the Ogoue River.These possessions were administered by the Spanish Viceroyalty of the Rio de la Plata whose capital was Buenos Aires.Spanish hopes settling the border would help economic growth were hampered by the 1779-1783 war with Britain that restricted trade with mainland Spain and led to high tariffs and taxes to pay for it.