 In 1571 Portugal, supported by the Kingdom of Kongo, took control of the south-western region of Angola in order to secure its threatened economic interest in the area.Although Kongo later joined a coalition in 1591 to force the Portuguese out, Portugal had secured a foothold on the continent that it continued to occupy until the 20th century.Despite these incidences of occasional violence between African and European forces, many African states ensured that any trade went on in their own terms, for instance, imposing custom duties on foreign ships.