Tacitus was a Roman historian whose descriptions of the Germanic tribes may have been intended as a foil, warning the Romans of their own weaknesses, failings. For this reason, his descriptions of Germanic society may be idealized.

Tacitus is credited with coining the term comitatus to describe the military structure of Germanic tribes.

From Germania (95 CE)
13. They transact no public or private business without being armed. It is not, however, usual for anyone to wear arms till the state has recognised his power to use them. Then in the presence of the council one of the chiefs, or the young man's father, or some kinsman, equips him with a shield and a spear. These arms are what the "toga" is with us, the first honour with which youth is invested. Up to this time he is regarded as a member of a household, afterwards as a member of the commonwealth. Very noble birth or great services rendered by the father secure for lads the rank of a chief; such lads attach themselves to men of mature strength and of long approved valour. It is no shame to be seen among a chief's followers. Even in his escort there are gradations of rank, dependent on the choice of the man to whom they are attached. These followers vie keenly with each other as to who shall rank first with his chief, the chiefs as to who shall have the most numerous and the bravest followers. It is an honour as well as a source of strength to be thus always surrounded by a large body of picked youths; it is an ornament in peace and a defence in war. And not only in his own tribe but also in the neighbouring states it is the renown and glory of a chief to be distinguished for. the number and valour of his followers, for such a man is courted by embassies, is honoured with presents, and the very prestige of his name often settles a war.

Comments
The comitatus was a retinue of warriors that attached itself to a lord or king voluntarily. Through oaths of loyalty, the comitatus protected militarily the lord or king who, in his turn, granted individuals the protection of the comitatus and rewarded them with wealth. The Germanic tribal economy was more or less identical to most tribal economies—it was primarily based on reciprocity rather than trade. In a reciprocal economy, goods and services are distributed as gifts in an expression of the social relationship and mutual obligations inhering between members of the group. The comitatus was a sophisticated military organization built entirely on the economic logic of reciprocity. --Hooker, Richard. The Middle Ages. 6 June 1999 <http://www.wsu.edu:8001/%7Edee/MA/GERMANS.HTM>.

The first century Roman historian Tacitus coined the term comitatus in his observations of Germanic tribal culture in Germania. The comitatus was a gathering of warriors under one governing lord, representing a strategic interweaving of family threads so as simultaneously to enlarge and secure tribal identity and allegiance. The comitatus and intermarriages among tribes were physical representations of intertribal treaties -- or rather the pre-literate versions of treaties, called friths, in which physical objects of value that represented the tribe were exchanged.

In much the same way, blood-money (wergild, or literally man-money) is offered by one tribe as payment for those of another tribe whom they kill. And tributes of swords and rings, necklaces and battle gear are offered as seals of good faith -- physical objects in place of (non-existant) written contracts. They are markers of agreements which, without writing, have no other physical representation. --Maxwell, Laura Maxwell. 1994 <http://www.thecore.nus.edu.sg/cpace/ht/knots/Comitatus_Tribal_ID.html>.

An interesting essay on the relationships within the comitatus: Mannerbund, Comitatus, and Gender.