Battle of Issus, 333BC
The first battle of Alexander the Great where Persian King Darius III was present
Initially Darius encamped in an open plain where his numerical superiority would be felt, lacking patience he marched into the coastal hills where his forces...

would lack sufficient terrain to properly maneuver & make their numbers felt
Instead the narrow coastal strip gave advantage to Alexander, whose army was smaller but of higher quality
In the battle Alexander & his cavalry attacked the Persian left wing, breaking through...

causing the whole Persian army to flee
Alexander captured Darius family & shattered Persian prestige
The entire western half of the Persian Empire fell into Alexander's lap, Egypt fell without a fight
Alexander & Darius would meet again at the battle of Gaugamela in 331 BC