The fyrd was the Anglo-saxon fighting force. From the beginning of the Anglo-saxon period around 410AD right through to 1066, the structure of the fyrd evolved but it’s main task was always the same, to fight wars and battles for their chieftain or king when needed.
The best evidence for the structure of a fyrd comes from the reign of Alfred the Great. Due to the Viking’s numerous victories and battle tactics, King Alfred needed to revolutionise the way he fought his battles. Beforehand, the fyrd was made up of mostly freemen like the ceorls, everyday farmers who could be called up to serve in the fyrd. The king and his nobles also had their own bodyguards and thanes who were more accustomed to war but their numbers were not enough to constitute an army entirely of their kind.
There was also a severe lag in time between a viking attack and assembling the fyrd. Once a viking raid was known a messenger would be sent to find his lord or the local aeldorman of the shire. How quickly this first leg in the process took depended on where the Aeldorman was at the time. If they were lucky he was at home but more often than not he was travelling between each of his estates managing his people or he could also be hunting or attending a witan or the king personally. Once the Aeldorman was found he would then have to send word to each of his estates and call up those of his freemen who were able to serve in the fyrd. By the time these men were assembled, marched to a meeting point, then marched to where ever the attack happened, the Vikings in their quick boats would have already done damage to a few towns and left.
When the great heathen army arrived in 865 and spent the next several years attacking the kingdoms of England, their success was as much due to their own fighting capabilities as the Anglo-saxon’s inefficient defences. The previous scenario could be played out one step further. Once the aeldorman had known about the attacks he most probably would have sent messages for his own fyrd to assemble as well as to his king. Again the messenger would have to travel all over the kingdom trying to find the king (as they did not have a central court but several royal vills or estates of their own), at which time the king having received the message would then have to send messages to his aeldormen, who would have to be found likewise, who would then send their own messages to their estates for the fyrd to be called up etc etc. All of this to and frowing could take weeks by which time the Vikings would have done much damage and were probably halfway back to Scandinavia with their loot. It was an inefficient system to say the least.
King Alfred’s idea was to introduce more professionally trained thanes and warriors as well as make service compulsory for every month out of three. This meant that at least a quarter of his people were ready and willing to defend at a moments notice, with the other three quarters back at home keeping their farms and estates until they were needed to fight. For a society whose primary industry was agriculture this was especially important. Many battles in the past had not only been lost because of failed military tactics, but because the men had to go home to tend to their farms (the most famous example was in 1066 when king Harold had to disband his men for the harvest despite the threat of William the conqueror making plans to attack England). The harvest season was one of the most important times of the year and if it was not brought in, the country would have no grain and vegetables for themselves or their lifestock for the coming winter or even the coming year. Alfred’s new scheme meant that he could both feed and defend his people.