Complex sentence: These groups managed to overwhelm nearly all of the loyal Roman outposts and settlements; the entire western and northern areas of Britannia were overwhelmed, the cities sacked and the civilian Romano-British murdered, raped, or enslaved.
Split into simpler sentences: These groups managed to overwhelm nearly all of the loyal Roman outposts and settlements. The entire western and northern areas of Britannia were overwhelmed, the cities sacked and the civilian Romano-British murdered, raped, or enslaved.