Early Saxon armies were indeed quite small; military service was a privilege extended to the thegn classes and the Freeman class of peasantry. The thegns formed complex strata of a landowning military elite who are the direct evolution of the gesith, the close retinue warband of the first warlord-kings. As kingdoms expanded and consolidated and bureaucratised, the close relationships centred around the type of personal gift-giving and feasting we see in Beowulf evolved into the granting of land tenure instead. This in turn lead to those thegns being able to recruit a gesith of their own, rewarding the service of their thegns with the grants of land within their own estate. Freemen, as the wealthiest class of free peasantry, owned their land outright, and so likewise had a vested interest in military service, usually in wars that were limited skirmishes for definite limited territorial gains or political authority.

The main change for this was the arrival of the Great Heathen Army in the 860s. While the West Saxon army actually performed very well against the Danes in pitched battles, a small, elite force simply didn't have the manpower and mobility to respond rapidly to mobile and distributed raiding forces and still face the main Danish army in battle. The English were forced either to remain permanently on the march, or distribute their forces in myriad small garrisons, which meant either facing the enemy exhausted or severely under-strength. Things come to a head in 871 where both Asser and the Anglo-Saxon Chronicle complain that West Saxon manpower was almost entirely used up in nine major battles, small expeditions against Danish camps, and "innumerable small skirmishes" against raiding bands. At the Battle of Wilton, the last major engagement of 871, Alfred's army goes into battle heavily outnumbered by the Danes. They score an early victory by surprising the Danish centre with a ferocious charge, but are unable to pursue for risk of finding themselves encircled by the Danish flanks, and instead are forced to withdraw, leaving the Danes in possession of the field.

What changes is the institution of the fyrd militia. The fyrd is created through a legal reform which extends the obligations of military service far further down the social hierarchy than ever before, to encompass all free peasantry rather than just the Freeman class. What this means in practice is that the "median thegns" and "thegns ordinary" who previously simply had the obligation to perform service in the gesith of their King's Thegn, and thus in service of the king directly, now had the obligation to raise their own gesith from amongst their tenants. This meant that thegns at a town, hundred or even village level would recruit a warband of the most willing and capable from among their tenants, and these small warbands would then coalesce at the new burh fortresses into a fyrd organised at shire level. This reform essentially created a standing army garrison at county level to deal with localised raiding and incursions, and freed up manpower for the king's army, although royal forces were often accompanied by detachments from each shire's fyrd as well, such as Alfred's army at Eddington in 879.