he Battle of Cannae was a disaster for Roman arms on the largest possible scale. The losses were comparable more or less to those at Trebia and Lake Trasumennus put together. Viewed in a broader historical context, the Roman losses at Cannae were comparable to the combined losses at Carrhae and the Teutoberg Forest, the two worst worst defeats suffered by the Romans at foreign hands in the period of the late Republic and early Empire. Indeed, one would have to go forward nearly six hundred years, to Adrianople in the late fourth century CE, to find a Roman military disaster of comparable magnitude.
The strategic defeat was equal in magnitude to the sheer loss of soldiers. Carrhae and Teutoberg Forest both took place at the edge of the Roman zone of control (and both set essentially permanent limits on Roman expansion in their respective regions). Trebia might also be said to have happened at what was then the edge of the Roman zone; of all the battles considered, only Lake Trasumennus and Adrianople itself might be said to have happened well within the contemporary Roman heartland. Moreover, both Trebia and Lake Trasumennus, disastrous as they both were, left substantial Roman armies still in the field, armies comparable in strength to those that had been lost, and even after Adrianople the empire still had other armies to call on.
Cannae, in contrast, left Rome effectively without an army in the field; the Romans had thrown practically everything they had at Hannibal, and he destroyed nearly all. To add to their trouble, only a few days after Cannae they got word that their army in Cisalpine Gaul had been ambushed and destroyed by the Gauls. At most ten thousand men escaped the battle and made their way to Venusia and Canusia, far too few even to constitute an "army in being," the more so since in view of their experience their battle-readiness must have been poor and their morale worse.
The phase of the w...