Actually, England was one of the few polities at the time which had something approaching the concept of a standing army. The fyrd was a semi-permanent garrison force recruited on a rotational basis from the general population during the campaign season and stationed at burhs, fortified enclosures (often but not always settlements) situated at prominent road junctions, ports, river crossings and the like. The fyrd is likely to have been an earlier concept - charter evidence suggests it exists as a concept during the reign of Offa of Mercia in the 8th Century - but comes to major prominence during the reign of Alfred of Wessex in the latter half of the ninth century during a major bureaucratic overhaul of the defence of England.

The fyrd is one of the trimoda necessitas - the necessary trinity - frequently attested to in later Anglo-Saxon charters. Grants of land by the king are often exempt from taxation beyond these trimoda necessitas, namely the need for landowners to supply manpower for the construction and maintenance of fortifications, the maintenance of roads and bridges, and for service in the fyrd. In any year, potentially a third of all men of fighting age were eligable for service. In an agrarian economy, the burden of maintaining these forces away from their farms was supported by a system known as the Burghal Hidage by which hides (approx. 120 acres) of land around each burh site were requisitioned to supply the garrison force. In the case of the Mercian burghal system which developed in the late ninth and early tenth century, a sophisticated logistical network maintained disparate garrison forces in strategically important but sparsely populated regions.

Despite being royally mandated, organisation of the fyrd was organised at the county level. Within each county, the ealdorman was responsible for the overall organisation of the defences of his county in line with the guidelines laid down by the king. Asser's Vita Ælfredi records that Alfred gave strict instruction that his ealdormen be sufficiently literate and numerate to carry out the bureaucracy necessary to organise the defence of their counties. At the Hundred (sub-county) level, organisation was further delegated to the level of thegns and hundredmen who would be responsible for the organisation of burhs, fyrd garrison forces and local defences in their region. In royal vils or tuns, a reeve - a royal officer - may have been responsible instead.

The English use of burhs and garrisons along major travel routes combined with regular surveillance sites and signalling beacons suggest that the fyrd may have been used for some law enforcement purposes. Broadly speaking, law enforcement was seen as a communal responsibility organised predominantly at the Hundred level and overseen by the local reeve and his retinue, or by the Hundredman. By the later tenth century, a system of Frankenpledge - essentially communal assurance groups - were also coming into effect.

The St Brice's Day massacre itself is actually attested to in a charter of King Æthelred II in 1004 to St Frideswide's Abbey in Oxford (S909 in the eSawyer database of Sawyer's Anglo-Saxon Charters) to repair damage done during the massacre. Unrepentantly boasting about having ordered it, Æthelred states "all the Danes who had sprung up in this island, sprouting like cockle amongst the wheat, were to be destroyed by a most just extermination." The actual massacre in Oxford took place at the hands of the urbanos suburbanosque, the people of the town and the suburbs. This has the rather unpleasant implication of a lynch mob turning on its neighbours in an act of popular ethnic violence. Given that the massacre was instigated by the king and his 'leading men' it's likely that these supposed mobs were at the very least supported by if not directly led by local elites and their retinues, and probably by fyrd forces.

It's worth mentioning that while the Anglo-Saxon Chronicle simply says that the massacre occurred across England, the only other solid record of it comes from a charter from Oxford. Oxford was not part of the old 'Danelaw', so the Danes there would have been itinerant traders. It's likely that the victims of the massacre in general were not the "Danish" aristocracy of the Midlands and North who had settled during the ninth century, and who had come to to a political compromise with King Edgar previously, but actual traders, mercenaries and/or envoys from Denmark (though we can't know for sure). Because royal control was extremely strong in the towns (burhs) where Aethelstans laws had mandated all trade occur, a massacre would have been relatively easy for Aethelred to orchestrate.