E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Capua corrupted Hannibal. Luxury and self-indulgence will ruin anyone. Hannibal was everywhere victorious over the Romans till he took up his winter quarters at Capua, the most luxurious city of Italy. When he left Capua his star began to wane, and, ere long, Carthage was in ruins and himself an exile.
Capua was the Cannæ of Hannibal. As the battle of Cannæ was most disastrous to the Roman army, so was the luxury of Capua to Hannibals army. We have a modern adaptation to this proverb: Moscow was the Austerlitz of Napoleon.