The West Indies were islands and small parts of the American mainland scattered around the Caribbean Sea. They were named the Indies because Columbus thought he had arrived in the Indian Ocean when he first sighted land in 1494. The area was dominated for many years by Spanish conquistadors on land and the Spanish navy at sea. Trade was forbidden to all non-Spanish ships, except those allowed by the Asiento de Negros. This regulated the slave trade from Africa into Spanish America and was largely in the hands of the Portuguese. Eventually, later in the 16th century, British, Dutch and French traders and privateers (licenced pirates) forced their way into the Caribbean to attack Spanish treasure fleets; they visited Spanish settlements for refuge and resupply as well as taking part in the slave trade. As the Spanish power weakened, these other nations began to establish a permanent presence in the so-called West Indies.