According to what I read in Livy's History of Rome, at least it didn't take the Romans years to raise a new army. In those years, Rome didn't have a standing army at all. Romans are citizen soldiers, not veteran mercenaries.
For example, only a few months after the conclusion of second Punic war, when the Romans determined to march against Philip, king of Macedon, and at the same time chastise the Gauls who ravaged northern frontiers, they first disbanded all their armies which had served in the former war, and authorized the two newly elected consuls to raise six new legions, two for Macedonian war, two to Gaul, and two as reserves. At the same time, several secondary officers such as praetors and propraetors were to raise a few small armies of several thousand men to act as garrison in several important provinces such Sicily.
All the deliberations, debations, prepararions were done in merely several months, and then the Romans were defeating Gauls and Macedonians at the same time.
So I really believe if we give at least the Romans in their Latin cities the ability of 0 turn recruitment, it will only be more historical. So the player will be able to raise a huge army in no time when face a serious war, and disband them as soon as the crisis is over, as the Romans did in history.
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