The massive expansion of the Oromo (or Galla as they were formerly known) at the beginning of the sixteenth century presumably was the result of multiple factors. One important impetus for the massive expansion of the Oromo might have been the gada system as the central institution of social and political organization, which ruled every aspect of life of the Oromo. An increase in population and demographic pressure also caused the leaving of former homelands of many of the Oromo groups. Oral traditions furthermore speak of great droughts at the time of their migration.
Originally there were two confederations of Oromo, called Borana and Barentu. During the first phase of their migration after 1530 the Oromo profited by the aftermath of the fierce warfare between Muslims and Christians and were advancing into regions virtually depopulated. The first land acquisition seems to have taken place as early as 1530 when the Borana Oromo invaded Bali. Between 1530 and 1538, in the period of the Mudana gada, the Oromo crossed the Wabi.
A tradition relates that even at this time a three-stage process could be discerned that consisted of scouting, surprise attack, and settlement. In the period of the Kiolole gada (1538-1546), they devastated the predominantly Muslim-populated country of Dawa’ro north of the Wabi river and the lowlands of the Hawas further north. The Christian emperor Galawdewos fought against the Barentu and the Borana during the Bifole Gada (1546-1554) but could not prevent them from attacking Waj, Fatagar, and other provinces.
Particularly important in the history of the Oromo migrations was the period of the Michelle gada (1554-1562), because it brought about a decrease in the power of both the Christians and the Muslims, which ended two centuries of struggle between the two groups. At the same time, the power of the Oromo dramatically increased, and Christians and Muslims united in their struggle against the common enemy for the next 150 years.
During the period of the Harmufa gada (1562-1579) the Oromo began migrating to the southwestern region of present-day Ethiopia. They attacked the provinces Angot, Amhara, and Bagemder, which were beginning to recover from the devastation of the war against the Muslims. Here the armies of the new emperor, Minas, confronted them. At the same time, Barentu Oromo groups attacked Adal, where famine and plague weakened any potential resistance. Only a few small groups of Muslims were able to flee to the Awsa Oasis in the Afar desert or survived within the fortified city. The greater part of the Muslim population of Adal was assimilated by the Oromo.
When Emperor Minas died in 1563, a third of the Ethiopian Empire was already occupied by Oromo groups. Their military strength was considerably improved by the introduction of the horse. During the first half of the seventeenth century, invasions by different Oromo groups were a permanent menace to the Ethiopian Empire. About 1617 the Borana attacked Bagemder and Gojjam, which were central regions of the empire. Between 1620 and 1660 the Ethiopian emperors had to constantly defend different parts of their territory but were unable to put a stop to the waves of advancing Oromo groups.
The main target of Oromo expansion was Gojjam, which was assaulted from the south by the Liban Oromo, and from the east by the Tulama. Simultaneously, the Tulama expanded from Shoa into Amhara and the Wallo and Azebo overran Angot, parts of Amhara and Waj, Bagemder, and Tigre. In 1642 the eastern Oromo nearly annihilated the Ethiopian army in Tigre.
Under the reign of emperors Fasiladas and Yohannes the Oromo seem to have been virtually unrestrained in their expansion. It was Iyasu I (1682-1706) who resumed the offensive against the Oromo while at the same time recruiting battalions of loyal Oromo groups whom he settled in conquered areas. Tulama and Liban Oromo were settled in northern Gojjam and Bagemder. They were encouraged to convert to Christianity. Some of their authorities were appointed to high offices in the army and in the administration of the provinces. In 1684-1685 Oromo groups fought against Emperor Iyassu I in Wollo and Gojjam. In 1694 the Gugru-Oromo attacked Gojjam and Bagemder.
Although the military expansion of the Oromo continued, many Oromo groups started to settle in Ethiopian territory and developed into a political power, which was utilized by the different secular and ecclesiastical groupings. By the end of the seventeenth century they were taking an active part in the political formation of the Ethiopian state. The process of mutual assimilation between the Oromo newcomers and other inhabitants of the empire was well under way.
Hassen, M. The Oromo of Ethiopia: A History 1570-1860.
Cambridge: Cambridge University Press, 1990.
Lewis, H. S. “The Origin of the Galla and the Somali.” Journal of African History. 7 (1966): 27-46.
Van de Loo, J. Guji Oromo Culture in Southern Ethiopia. Berlin: Dietrich Reimer Verlag, 1991.