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
If nevertheless no special soldier-class sprang up,
but on the contrary the army still remained, as before, a burgess
army, this object was chiefly attained by abandoning the former mode
of ranking the soldiers according to property(22) and arranging them
according to length of service.
The Roman recruit now entered among
the light-armed "skirmishers" (-rorarii-), who fought outside of the
line and especially with stone slings, and he advanced from this step
by step to the first and then to the second division, till at length
the soldiers of long service and experience were associated together
in the corps of the -triarii-, which was numerically the weakest but
imparted its tone and spirit to the whole army.
The excellence of this military organization, which became the primary
cause of the superior political position of the Roman community,
chiefly depended on the three great military principles of maintaining
a reserve, of combining the close and distant modes of fighting, and
of combining the offensive and the defensive.