Not a historian but i'll try to answer this the best way i can, if an actual historian comes along and answers i'll be more than happy!

Over the course of the 4th and 5th centuries AD, the western (and sometimes eastern) roman empire gave out some lands along their frontiers to various germanic tribes in exchange for those same tribes protecting the frontiers and contributing men to the roman armies.

In the latter half of the 5th century, the western roman empire fell, and in it's wake left a power vaccum which was very quickly filled by the new germanic kingdoms of europe, the kingdom of odoacer/ostrogoths in italy and illiricum, the visigoths in hispania and aquitaine, the franks in north-eastern gaul and the burgundians in provence/arelatum.

Initially the fall of the western roman empire did not have a great impact, most germanic kings would surround themselves with roman or romanized aristocratic advisors, highly prized for their literacy, over time though these roman advisors would die, taking with them their literacy, this is not to say that literacy died out, it survived thanks to the ranks of the church but general literacy was greatly reduced. With reduced literacy came an inability to conduct bureaucracy, you can't keep records of thousands of settlement, let alone a handful if your clerks can't even write down simple things.

So instead a simpler system was adopted, based on the concepts of germanic kingship, the kings would divide their kingdoms into territories known as counties, usually these would be based upon the locations of cities, the counties would be granted to member of the martial nobility, and from these counties the king would be able to raise troops, and levy taxes.

Of these apply to larger kingdoms, like the frankish kingdoms of the merovingians and carolingians and the visigoths, and even within those kingdoms there were exceptions.

The early merovingian monarchs maintained the legions along the rhine well into the 6th century. In addition after the conquest of burgundy, the merovingian monarchs maintained the military institutions of the former kingdom, mroe specifically a standing professional army. In addition the vascones and gallo roman aquitanians, both maintained standing armies. Finally during the 8th century, the carolingian monarchs revived the tradition of standing armies, although to a lesser extent than the roman antecendents.

I hope this helps you somewhat, have a good one!