Complex sentence: Brennus (or Brennos) was a chieftain of the Senones, a Gaulish tribe originating from the modern areas of France known as Seine-et-Marne, Loiret, and Yonne, but which had expanded to occupy northern Italy.
Split into simpler sentences: Brennus (or Brennos) was a chieftain of the Senones. The Senones were a Gaulish tribe originating from the modern areas of France known as Seine-et-Marne, Loiret, and Yonne, but which had expanded to occupy northern Italy.