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
Origin of the Manipular Legion
That this new military organization was in the main a Roman, or at any rate Italian, remodelling and improvement of the old Greek tactics of the phalanx, is plain. If some germs of the system of reserve and of the individualizing of the smaller subdivisions of the army are found to occur among the later Greek strategists, especially Xenophon, this only shows that they felt the defectiveness of the old system, but were not well able to obviate it.
The manipular legion appears fully developed in the war with Pyrrhus; when and under what circumstances it arose, whether at once or gradually, can no longer be ascertained. The first tactical system which the Romans encountered, fundamentally different from the earlier Italo-Greek system, was the Celtic sword-phalanx. It is not impossible that the subdivision of the army and the intervals between the maniples in front were arranged with a view to resist, as they did resist, its first and only dangerous charge; and it accords with this hypothesis that Marcus Furius Camillus, the most celebrated Roman general of the Gallic epoch, is presented in various detached notices as the reformer of the Roman military system.
The further traditions associated with the Samnite and Pyrrhic wars are neither sufficiently accredited, nor can they with certainty be duly arranged;(23) although it is in itself probable that the prolonged Samnite mountain warfare exercised a lasting influence on the individual development of the Roman soldier, and that the struggle with one of the first masters of the art of war, belonging to the school of the great Alexander, effected an improvement in the technical features of the Roman military system.
23. According to Roman tradition the Romans originally carried quadrangular shields, after which they borrowed from the Etruscans the round hoplite shield (-clupeus-, --aspis--), and from the Samnites the later square shield (-scutum-, --thureos--), and the javelin (-veru-) (Diodor. Vat. Fr. p. 54; Sallust, Cat. 51, 38; Virgil, Aen. vii. 665; Festus, Ep. v. Samnites, p. 327, Mull.; and the authorities cited in Marquardt, Handb. iii. 2, 241).
But it may be regarded as certain that the hoplite shield or, in other words, the tactics of the Doric phalanx were imitated not from the Etruscans, but directly from the Greeks, As to the -scutum-, that large, cylindrical, convex leather shield must certainly have taken the place of the flat copper -clupeus-, when the phalanx was broken up into maniples; but the undoubted derivation of the word from the Greek casts suspicion on the derivation of the thing itself from the Samnites. From the Greeks the Romans derived also the sling (-funda- from --sphendone--). (like -fides- from --sphion--),(Cf. I. XV. Earliest Greek Influences). The pilum was considered by the ancients as quite a Roman invention.
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Reference address : https://www.ellopos.net/elpenor/rome/2-08-law-religion-army-economy-nationality.asp?pg=23