Complex sentence: The territory which is now Norway was first settled in 12,000 BC following the retreat of the great inland ice sheets and the Neolithic period started 4000 BC. The Migration Period caused the first chieftains to take control and the first defenses to be made.
Split into simpler sentences: The territory which is now Norway was first settled in 12,000 BC following the retreat of the great inland ice sheets. The Neolithic period started 4000 BC. The Migration Period caused the first chieftains to take control and the first defenses to be made.