Elephants in War
The tactical use of elephants, originating earlier in India, was brought forcibly to the Western military mind at the Hydaspes, where both the potentialities and the limitations of the war elephant were demonstrated. Alexander’s horses refused to face the beasts, yet his disciplined phalanx, despite initial surprise and dismay, eventually turned the elephants back in panic-stricken flight. Seleucus’ impressions, however, were sufficiently favorable for him—20 years later—to cede substantial territory to Chandragupta in exchange for 500 elephants, which he proceeded to use to advantage in his victory at Ipsus. After that time the use of the war elephant spread rapidly to Greece and Carthage. That the beast was a valuable weapon is clear from its use by such objective warriors as Hannibal and Pyrrhus….
As proven at Beneventum and Heraclea, elephants were most successful when used against troops unacquainted with them. Disciplined and resourceful opponents, however, could stampede the elephants, which then became more dangerous to friend than foe. For this reason, the war elephant’s mahout (driver) carried a steel spike to hammer into the beast’s brain should he stampede.
— The Encyclopedia of Military History, p. 40