Legionary Training

[visegrip]

I'm curious, in much of my reading of the Roman legions. It seems that at least in Caesar's era, Legions were recruited en masse, and the casualties weren't replaced, so a legion after some years of campaigning could be massively understrength.

What did they do when the Legion became horrifically understrength? When did Rome adopt the replacements idea to put new troops in to depleted units, and why so long to adopt that practice?

Chris