Like most African groups of any size, the Ovimbundu were formed by the mixture of groups of diverse origin (and varying size). Little is known of developments before the seventeenth century, but there is some evidence of additions to the people who occupied the Benguela Plateau at that time. Over time, a number of political entities, usually referred to as kingdoms, were formed. By the eighteenth century, there were twenty-two kingdoms. Thirteen were fully independent; the other nine were largely autonomous but owed tribute to one of the more powerful entities, usually the kingdom of Bailundu, but in some cases Wambu or Ciyaka. By the beginning of the second decade of the twentieth century, effective occupation by the Portuguese had caused a fairly rapid decline in the power of the heads of these kingdoms, but Ovimbundu continued to think of themselves as members of one or another of the groups based on these political units after World War II.