Complex sentence: By the 5th century the Angles, after whom East Anglia and England itself are named, had established control of the region and later became the "north folk" and the "south folk", hence, "Norfolk" and "Suffolk".
Split into simpler sentences: By the 5th century the Angles (after whom East Anglia and England are named) had established control of the region. The Angles later became the "north folk" and the "south folk", from which developed the names "Norfolk" and "Suffolk".