The Oyo Empire was the Yoruba cavalry state that dominated the savanna of what is now southwestern Nigeria and the Republic of Benin from roughly the sixteenth to the early nineteenth century. It built its power on horses across open grassland, taxed trade toward the Atlantic coast, and bound its sacred king, the alaafin, inside a web of constitutional checks so strong that a council could, by custom, require him to take his own life. Its early-nineteenth-century collapse fed the Yoruba civil wars and a surge of enslaved Yoruba carried to the Americas.
By Oyo's political tradition, when its council of chiefs judged that the king had failed, they did not depose him with an army. They sent him a gift. An empty calabash, or a dish of parrots' eggs, arrived at the palace, and everyone understood the message. The alaafin was a god on earth, the link between his people and the heavens, and yet seven men could send him a basket that meant his reign and his life were over. By custom he was expected to take his own life. Here was a sacred king hedged about with rules that could command his death, and the men who held that power were as much a part of the state as the king himself.