When we think of the Anglo-Saxon Period, we often picture roving, bloodthirsty bands of warriors who went around raiding helpless villages. In actuality it wasn't just random tribes that existed back then; there was a relationship between them - they all followed comitatus. Comitatus is the unwritten law in which a warrior pledges his allegiance to a lord in exchange for protection, shelter, and a place in his mead hall. The mead hall was where the warriors, or liegemen, of the lord gathered to eat and drink (mostly drink) in the evening and swap war stories 'till the wee hours of the morning. Many of the men didn't have family and were just lonely mercenaries until a lord picked them up and gave them a home. Comitatus was what held Anglo-Saxon societies together.
A prime example of comitatus is in the old pagan story Beowulf . It tells of a great Geat warrior, Beowulf, who learns about a horrible monster, Grendel, and decides to go slay him. The land Grendel is ravaging is under the rule of King Hrothgar. One day Beowulf shows up and says to Hrothgar, "I have come so far, / Oh shelterer of warriors and your people's loved friend, / That this one favor you should not refuse me- / That I, alone and with the help of my men, / May purge all evil from this hall." With these few words Beowulf swore allegiance to Hrothgar until either he or Grendel was dead. This was a oath he'd keep with his life, to someone he barely even knew. This loyalty to anyone you gave an oath to made sure that the Anglo-Saxon world didn't slide into anarchy.
The Wanderer, another Anglo-Saxon story, is about a man put into exile when his lord dies. This man is so overcome with his lord's death that he does nothing but reminisce about the fun he had at the mead hall with his warrior buddies and his lord. He has no idea what is going on at the present time. He can only think about his misery at his exile and try to become like a stone and let everything pass over him while he mourns his lord. His sorrow continues even in sleep, "Even in slumber his sorrow assaileth, / And, dreaming he claspeth his dear lord again, / Head on knee, hand on knee, loyally laying, / Pledging his liege as in days long past." He is totally consumed with loyalty to his lord and most of him died with his lord. That is how deep comitatus was seeded in some people during that time.
In Morte d'Arthur the way comitatus is carried out has changed. Warriors are not allied to their lord - they're allied to their king. And, the warriors are now mostly knights, and the idea of comitatus is now the tradition of courtly love, or chivalry. The loyalty now also lays in loyalty to lady as well as lord. This excerpt will show Sir Lucan's loyalty to an injured King Arthur. "Then Sir Lucan took up the King the t'one (side) and Sir Bedivere the other (side); and in the lifting up the King swooned and in the lifting Sir Lucan fell in a swoon that part of his guts fell out of his belt, and therewith the noble knight's heart burst. And when the King awoke he beheld Sir Lucan how he lay foaming at the mouth and part of his guts lay at his feet." Sir Lucan just killed himself taking care of Arthur. He never uttered the smallest moan even though he was hurt much worse than the king. The scene has changed, but the bond is just as strong.
I find it strange that so many peoples across such a large area found a way to have the same type of society rules, like comitatus. There wasn't much information sharing in those days between villages, much less between tribes. The odd ship or traveller would bring news and rumor of the nearest neighboring tribe, and that was about it. Something in their subconscious must have told them there should be order and they discovered it in the form of comitatus.