Complex sentence: Racial separation, including residential segregation, was enforced as thoroughly as possible and the Italians showed favouritism to non-Christian groups such as the Oromos, the Somalis and other Muslims (many of whom had supported the Italian invasion).
Split into simpler sentences: Racial separation, including residential segregation, was enforced as thoroughly as possible. In addition, the Italians showed favouritism to non-Christian ethnicities such as the Oromos, the Somalis, and other Muslims (many of whom had supported the Italian invasion).