They told the people Oromo was abused and never had power. Abiy, partly non-Oromo, like Meles Zenawi who was 50% from Eritrean Tigre, rode the Oromo who were made believe they were abused by Amhara ethnic (a flawed hypothesis) and usurped power in 4-Killo. He killed millions of Tigrians, now Amhara in hundreds of thousands and Oromos did not rest, they also die like fly.
Mengistu was presumed to be Amhara (but debated, some even say he was southerner or of Oromo descent) but killed everybody.
The kings, perhaps with good intent, or may be with some facts, proclaimed they were Solomonic dynasty. They did what they wanted to do. Mostly they succeeded to usher peace and unity (gave us a country at least, they did not discriminate in ethnic ground despite hiccups), a lot better still with exploitation by the nobility.
Even go to Eritrea, one psychopath convinced Eritreans that they need freedom or abolish slavery from Ethiopia, they died, killed and succeeded to be alone but they are under one man wish without even constitution. He is a lifetime dictator, and the Eritreans wish to leave the land even to die and have already left the country in droves.
So, the fault is in the POLITICAL RHETHORIC used by the power seekers. The fault is in the people who believe them and let themselves and others to die. Our people need is not division. Not who is what ethnics. Ethnic hair splitting for political reason will never give us peace. This is a life we don't live forever. We need to coexist. We have enough resource and manpower. We have to somehow usher good governance (our enemies won't want that for us), taper down ethnic division (we survived together for millennia) and change the political rhetoric. otherwise, we will remain used, abused, exploited, killed and may have to kill our brothers. We have to think when to protect our brother. Say I am a defender of my brother. I don't know your ethnic, but if I meet you in some place and you need help, it will be difficult for me not to provide that help to you, first because you are human, second you are from Africa, third you may be from Ethiopia (not bs you are Oromo, Tigre Bla-Bla....). We need to wake up. We are used by politicians all our life
