What is the Comitatus and how does it fail Beowulf?

In the poem "Beowulf" the Comitatus is a group of comrades
or men who owe their allegiance to the chief or person they follow.  It is derived from
a Roman term.  Under the expectations, the men of the person would never allow the
leader to suffer needlessly because they would protect
him.

Beowulf's men let him down in several ways throughout
the poem.  One of the first experiences is when they fail initially to help him to
defeat Grendel.  Beowulf later recants and decides that he should be the one to kill
Grendel.

When Beowulf has to fight Grendel's mother, once
again Beowulf is on his own.  Last of all he is killed when he should have had his men
at his side to save him to help him slay the dragon. He has to slay the dragon on his
own after he is mortally wounded.