Introduction
The last couple of decades of the 11th century and the start of the 12th century are rather interesting in terms of recruitment from the peasantry. While the Normans had conquered England and co-opted a significant part of the pre-existing Anglo-Saxon military system, there was still quite a strong tradition of military service among the peasantry until the reign of Henry I. In fact, in many ways it was the English infantry who enabled the early Anglo-Norman kings to maintain such a high level of centralised power in comparison to other kingdoms at the time.
However, as interesting as the subject is, I should stress here and now that we don't have enough information to be entirely certain on how everything was structured. Between various charters and chroniclers we can make out a reasonable amount about recruitment, and various artworks from the period provide some information about equipment, but there is still a lot we don't know and which will be speculative in nature.
Recruitment
The first thing you, as a peasant in England around 1100 AD, would think if a baron told you he was raising an army on behalf of the king would probably be something along the lines of "Christ's teeth, you traitorous French bastard, do you really think you're going to get away with this?". The reason for this is that barons in medieval England had no right to raise armies. While they owed their own service and the service of a number of knights for a set period (possibly originally 60 days, later modified to 40, although with a number of variants on this theme), the right to raise an army - and the peasantry more specifically - belonged to the king alone, and for William II Rufus and Henry I, the English peasantry formed the backbone of their armies against baronial revolt in England and ultimately allowed them to consolidate power and keep England as a relatively centralised state.
This doesn't mean that the barons couldn't raise infantry from their own holdings, since personal relationships and patronage were key to military service in the Middle Ages and there would doubtless have been some freemen who were tenants or owned land in one of their villages who would see some advantage in serving their lord, would be willing to serve for pay or might have felt obligated to serve because they had a position in the baron's household or village (the village bailiff, for example), but it is important to keep in mind that these weren't men they could legal call up for service.
Generally speaking, the process of recruitment appears to have been that the king would send out a summons to arms to the greater magnates (earls, the more powerful barons and powerful churchmen) and to the county sheriffs. The country sheriffs, acting as the king's agents, would then issue a summons to the lesser barons and knights, and also summon those peasants liable for military service. There are some variations to this process, especially on the Welsh Marches, where the earls had a large degree of military independence, but it would be fair to say that most armies were summoned by either sheriffs, who were the agents of Royal power, or the most important personages of the area, with whom the king either had a significant personal relationship, needed to provide them with respect or where they had been given a large degree of control over military matters in their sphere of influence.
The Royal control over the recruitment process, and the fact that free peasants technically owed their ultimate allegiance to the King rather than whichever baron or knight owned their manor, is probably partially responsible for the significant support of the English for William the Conqueror during the earl's rebellion of 1075, their support of William II Rufus during the 1088 aristocratic rebellion and Henry I during Robert Curthose's invasion of 1101 and the rebellion of the Earl of Shrewsbury in 1102. In all these cases, it was the English and lesser knights (milites gregarii) who provided almost all of the royal armies, with the barons and other aristocrats forming the bulk of the rebellious forces. Doubtless an antipathy towards the conquering classes and the possibility of local tyrannies springing up was also a major consideration for those English who served the kings, but the fact that they were legally only bound to serve the king must also have been on their minds.
The royal control was also necessary, since William the Conqueror had deliberately fragmented land holdings when he divided England up so that no baron had too much land close together, most likely to prevent them from dominating any one area. The rich abbey of Peterborough, for example, had its lands spread out over seven separate counties, which would have greatly slowed down the assembling of the army. However, with each county's sheriffs calling up the scattered knights and infantry, the army could be more swiftly assembled.
So who were the Englishmen who fought for the early Anglo-Norman kings? They were, undoubtedly, free men, and this puts something of a limit on who could fight. Much of England's peasantry were, technically, unfree and were bound to their land. Of the rural peasantry recorded in the Domesday Book, around 40% were classed as villani (serfs), who typically held 15 acres or more, with another 30% who were classed as cottagers, who typically held 5 acres or less. Servi (technically slaves, although in some areas they were more like serfs bound to a specific household rather than a specific manor) and miscellaneous categories account for around 15%. Only 15% of the rural population, therefore, was actually free and liable for military service.
However, "free" does not necessarily mean "landholder with comfortable acreage". Just as there were cottagers who held 15 acres, there were free men who held less than most cottagers. Generally speaking, the less densely populated an area was, the more land individuals held, although this was by no means universal - in Middlesex (a fertile area in southern England), for example, almost one third of the villani held between 1 virgate (30 acres) and 2 virgates (60 acres), at a time when 15 acres was enough to sustain a family. Further, at this point in English history the line between "serf" and "free" was still quite fluid, with some free men subject to obligations to their lords (although less onerous than the labour of the serfs), while serfs could attain essentially free status by paying a cash rent. As such it's probable, although by no means certain, that some serfs also served in military endeavors, especially those who were particularly wealthy or lived in border regions. It's also worth noting that, while the disparities in landholding are most apparent in free landholders, they still held 20% of the land despite only making up 15% of the population, so those who held more land than average held a lot of it.
This restricts the pool further. Only those who own enough more land than needed to support themselves in a bad year are going to have the surplus needed to purchase the necessary equipment for fighting. We're fortunate in that we have exceptionally detailed records for the abbey of Peterborough compiled between 1113 and 1135, which not only record the knight-service owed, but also sub-feudal infantry service owed by tenants. Approximately 76 sokemen owed military service to the abbey, and this was only a small portion of the total sokemen on abbey estates. Although we don't know how much land each sokeman owned, the lands belonging to the abbey would suggest a military obligation in Anglo-Saxon times of 70 men and, since the abbey had also over-enoffed knights in what was most likely a policy of ensuring there were reserves in case of injury, old age or minority on some of the knight's fees they owed, the case was likely similar here.
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