Defence in depth in order to retreat successfully, and they assume that the defender's morale will recover from the retreat. A possible early example of this came at the Battle of Cannae in 216 BC, when Hannibal employed this manoeuvre in order to encircle and destroy eight Roman legions, but that is disputed by some historians. Edward Luttwak used the term to describe his theory of the defensive strategy employed by the Late Roman army in the 3rd and 4th centuries AD. Later examples of defence in depth might be European hill forts and the development of concentric castles. In those examples, the
Socii to raise a double-strength army of four legions and four "alae" e.g. at the Battle of Cannae in 216 BC, where each Consul commanded an army of about 40,000 men. Polybius states that the Romans and their allies could draw on a grand total of 770,000 men fit to bear arms (of which 70,000 met the property requirement for cavalry) in 225 BC, shortly before the start of the Second Punic War. The Romans reportedly asked their allies for an urgent register of all "men fit to bear arms" for a "tumultus Gallicus". Polybius' subtotals, however, are garbled, as he