Please note that Mommsen uses the AUC chronology (Ab Urbe Condita), i.e. from the founding of the City of Rome. You can use this reference table to have the B.C. dates
From: The History of Rome, by Theodor Mommsen
Translated with the sanction of the author by William Purdie Dickson
They developed the system of retainers, that is, the privilege of the nobility to surround themselves with a number of hired mounted servants-- the -ambacti- as they were called (18)--and thereby to form a state within the state; and, resting on the support of these troops of their own, they defied the legal authorities and the common levy and practically broke up the commonwealth.
18. This remarkable word must have been in use as early as the sixth century of Rome among the Celts in the valley of the Po; for Ennius is already acquainted with it, and it can only have reached the Italians at so early a period from that quarter. It is not merely Celtic, however, but also German, the root of our "Amt," as indeed the retainer-system itself is common to the Celts and the Germans. It would be of great historical importance to ascertain whether the word--and so also the thing--came to the Celts from the Germans, or to the Germans from the Celts. If, as is usually supposed, the word is originally German and primarily signified the servant standing in battle "against the back" (-and-= against, -bak- = back) of his master, this is not wholly irreconcileable with the singularly early occurrence of this word among the Celts.
According to all analogy the right to keep -ambacti-, that is, --doouloi misthotoi--, cannot have belonged to the Celtic nobility from the outset, but must only have developed itself gradually in antagonism to the older monarchy and to the equality of the free commons. If thus the system of -ambacti- among the Celts was not an ancient and national, but a comparatively recent institution, it is--looking to the relation which had subsisted for centuries between the Celts and Germans, and which is to be explained farther on--not merely possible but even probable that the Celts, in Italy as in Gaul, employed Germans chiefly as those hired servants-at- arms. The "Swiss guard" would therefore in that case be some thousands of years older than people suppose.
Should the term by which the Romans, perhaps after the example of the Celts, designate the Germans as a nation-the name -Germani---be really of Celtic origin, this obviously accords very well with that hypothesis.--No doubt these assumptions must necessarily give way, should the word -ambactus- be explained in a satisfactory way from a Celtic root; as in fact Zeuss (Gramm. p. 796), though doubtfully, traces it to -ambi- = around and -aig- = -agere-, viz. one moving round or moved round, and so attendants, servants. The circumstance that the word occurs also as a Celtic proper name (Zeuss, p. 77), and is perhaps preserved in the Cambrian -amaeth- = peasant, labourer (Zeuss, p. 156), cannot decide the point either way.
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Reference address : https://www.ellopos.net/elpenor/rome/5-07-subjugation-west.asp?pg=28