Wolves from the Sea: The Dark Ages

This book covers the Early Medieval period in western and northern Europe, commonly known as the "Dark Ages", from 496 AD until 1070 AD.
Following the collapse of the Western Roman Empire at the end of the 5th century, the post-Roman world was divided into a number of kingdoms, each ruled by the "barbarian" grouping that had come to control it during the last days of die Empire. After the initial round of wars, the Franks held most of modern France and also controlled a substantial area of Germany, the Visigoths held Spain, the Ostrogoths held Italy and the Vandals held North Africa.
Continuity with the Roman Empire, however, was at first considerable. Many of the "barbarian" armies, though of Germanic tribal origin, were in fact originally Late Roman field armies, and continued to use Late Roman methods of supporting the soldiers. Each soldier was assigned to share a portion of the land of a Roman landowner, from which he drew his pay.
In the mid 6th century the Eastern Roman (Byzantine) Empire launched a counter-offensive, reconquering Africa from the Vandals, Italy from the Ostrogoths and part of southern Spain from the Visigoths. Soon after, however, diey lost the north of Italy to the invading Lombards.