Committing the triarii meant that the hastati and principes had already been pressed back and the legion's final line was threatened. One interpretation of stories about charges without orders is that they softened the admission that a general had nearly lost the battle. Veteran officers could have recognized an approaching collapse and chosen to counterattack before the line gave way. Their special standing within the legion allowed considerable initiative. Some could also have held senior public offices, giving them experience and authority independent of the current general.
Notable Point · 2:00:40-2:03:24
Why accounts of triarii charging without orders need scrutiny
Stories of unauthorized triarii charges can be read as experienced officers intervening before collapse while sparing a general an admission that he needed his last reserve.