The Oromo are the largest ethnic nation in Ethiopia and are also present in northern Kenya, a Cushitic speaking people whose sixteenth century expansion across the Ethiopian highlands and Horn of Africa is documented in Mohammed Hassen's history of the period from 1570 to 1860. That expansion was organized through the gadaa system, an age grade structure that moves every Oromo man through eight year stages of herding, warfare, government and elder counsel, with a Luba grade holding executive power for a single eight year term before handing it on.
Gadaa assemblies, above all the one held at Gumii Gaayoo, made law, judged disputes and could depose an abba gadaa who governed badly, a check UNESCO's 2016 inscription of the system describes as a rare indigenous constitutional order rather than a monarchy. Ethiopian imperial conquest under Menelik II in the late nineteenth century absorbed Oromo lands into a centralized state that suppressed gadaa governance and the Oromo language in public life for a century afterward.
Zemene Mesafint era rivalries, forced settlement and land dispossession under successive Ethiopian governments left a legacy of political grievance that shaped the Oromo protest movements of the 2010s. Gadaa assemblies still meet among the Borana Oromo of southern Ethiopia and northern Kenya, and the practice has drawn organized revival efforts elsewhere since Oromo language rights expanded after 1991.