It's not like one day people just decided to seriously redo their whole military system of course. Especially the "Camillian-Polybian" shift was just a normal gradual evolution of equipement, tactics and organisation. It was always a rather lenghty process naturally - with the "Marian" developement the old citizen milita eventually became an increasingly unsustainable system, with ever more men simply hired and equipped by their employers to make up the balance... until you basically had the Republic full of "strong men" with their own private armies. Augustus was the guy who was left standing after the brown stuff hit the fan over than, and he understandably consolidated the multifarious more-or-less private forces into a rather more manageable, dependable and affordable core of standing "national" armies.
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