According to Richard Pankhurst, a British-born Ethiopian historian, this migration is linked to the first incursions into inland Horn of Africa by Imam Ahmad ibn Ibrahim. According to historian Marianne Bechhaus-Gerst, the migration was one of the consequences of the fierce Ethiopian–Adal war which killed a lot of people and depopulated the regions near the Galla lands, but also probably a result of droughts in their traditional homelands. Further, they acquired horses and their "gada" system helped coordinate well equipped Oromo warriors who enabled fellow Oromos to advance and settle into newer regions starting in the 1520s. This expansion continued through the 17th century.