The Romans followed up and captured Tunis, only from Carthage. In despair, the Carthaginians sued for peace, but Regulus's proposed terms were so harsh the Carthaginians decided to fight on. They gave charge of the training of their army to the Spartan mercenary commander Xanthippus. In the spring of 255 BC Xanthippus led an army of 12,000 infantry, 4,000 cavalry and 100 war elephants against the Romans' infantry-based army at the Battle of Tunis. The Romans had no effective answer to the elephants, their outnumbered cavalry were chased from the field and the Carthaginian cavalry then surrounded most of the Romans and decisively defeated them. Most of the Romans were killed, while approximately 500, including Regulus, were captured; another 2,000 Romans escaped and retreated to Aspis which was situated on a high and naturally strong position, and overlooked the natural harbour of the Bay of Clupea. Xanthippus, fearful of the envy of the Carthaginian generals he had outdone, took his pay and returned to Greece.