On Oromo civilizations of the past, read this link:
The so called Oromo migration is what we all did-conquer new lands, to have farm lands and water resources, to feed our people. Part of it was also the Gada system, every 8 years, the warriors had to seek new conquests, and fight, so part of that entailed conquering new lands. but the point was, hey, we have all this cattle/livestock, and need lands and water to farm, so let's go conquer some stuff.
somalis were like :''brazer, we need land and water since we are nomads, so let's go conquer some stuff.''
habeshas were like ''lol yo, our land has been devastated by war, and our elites are too lazy to farm and work, so let's beg the white man for guns, to conquer other africans, so they can work and feed us.''