Complex sentence: Members of the Tigrayan ethnic group constitute a minority of Ethiopia's population but dominate the senior positions in the country's military and political system while the majority Amhara and Oromo people are marginalized.
Split into simpler sentences: Members of the Tigrayan ethnic group constitute a minority of Ethiopia's population. However members of this ethnic group dominate the senior positions in the country's military and political system while the majority of Amhara and Oromo people are marginalized.