During this period the Carolingian Empire was the leading military power in the West stretching over France and Western Germany, then subsequently into Northern Italy and expanding its reach with campaigns into eastern Germany.
It has been calculated, by Contamine that Charlemagne was able to call on large number of direct vassals – perhaps 1,800 in total. These were the Counts, Abbots, Bishops, who administered his Empire and in return owed him particular war service (all freemen were liable to serve if the country was invaded). Contamine believes that each of these direct vassals of the King would be bound to contribute perhaps 20 housemen (sources who that these counts and bishops could often call on 30 vassals of their own). Thus making a total army size of 36,000 possible, however, a smaller army would usually be quite sufficient.
After 806 way of recruiting seems to have become more structured – with men being summoned depending on their landholding – as follows:
All those with benefices should come – i.e. the abbots and bishops and counts
Freemen with three or more manses should join the army, and for those with two manses – of every two men the one more competent in arms would have to go. For those with only half a manse, 6 men select one of their number and helped arm him. This rather complex system seems to have lasted from 807 to 825 at least.
In case of invasion all freemen were liable to be called up, this was the bannum or lantweri as it was known in German speaking parts of the Empire. For those who did not fight there were obligations to provide livestock, wagons and forced labour to build or repair fortifications.
In the Carolingian armies the Franks formed largest component, but Burgundians, Bavarians, Provençals, Occitans as well as Lombards were important elements as well.
Cavalry in this period were becoming more important. Light cavalry were recruited from Visigothic refugees from Muslim controlled Spain. Lombards were said to be the best cavalry by 9th century. Light cavalry were often drawn from border areas – Basque country, Brittany, Frisia and the newly conquered areas of eastern Germany.
On eastern frontier the fortifications manned by local professional soldiers, or warda, where a permanent force would be required.
In the South of France rather than raising soldiers by land grant, a system of professional milites existed, perhaps a remnant of old Roman systems, fighting for pay rather than land grant. Here Gascons good reputations as mercenary troops by 8th century.
Recruitment after the Decline of the Carolingian Empire and During The Time of the Vikings
By the time of the Viking incursions during the reign of Louis the Pious (814 to 840) the use of the general levy and of light cavalry had declined. Indeed it seems that the only troops capable of standing up to the threat were the heavy armoured cavalry – often fighting for a local feudal lord in exchange for land and protection. With the disintegration of central Royal authority, these local feudal lords came to the fore, expanding their territories in the regions through the use of heavily armed private armies. These armies acted almost as joint venture companies – with soldiers looking for successful lords who could lead them to victory and reward them well. These soldiers would one day become the knights of the Middle Ages.
During the later part of the period the French Kings began to claw back their lost authority, and did this during the 10th and 11th centuries by recruiting mercenaries to garrison castles and form armies in struggle to restore their authority.
Beginning of a New Age
Through the rise and fall of the Carolingians a new feudal age arose in the heart of Western Europe – France and Western Germany, and with it a rapid change in the way of recruiting armies. No longer was it possible for kings to gather massive armies as Charlemagne had done – the vassals were much more independent. Armies had to be brought together for pay and reward, or by the exertion of direct feudal service.
Contamine, Philippe, War in the Middle Ages
- Why did Charlemagne’s empire divide (wiki.answers.com)