The Battle of Carrhae
Plutarch, Life of Crassus, 24.5 (translated by Bernadotte Perrin)
As narrated by the Greek historian Plutarch (d. 120 CE), the Persians surrounded the Romans, who mainly consisted of infantry, with mounted archers who were supplied with a camel train full of arrows. As soon as the Roman infantry advanced towards them, the cavalry galloped away, firing more arrows behind them as they fled (a signature Parthian move which became known as the Parthian shot). When a body of troops under the command of Crassus' son Publius chased after the Parthians, they were cut off and quickly killed.
The Parthians did not just use archers, however. They also relied on armored horsemen with long spears, who kept the Romans hemmed in and finally broke the survivors:
Plutarch, Life of Crassus, 25.7, 27.1
Thrown into confusion, the Romans attempted to retreat to Roman territory. However, the Parthians pursued them, killed Crassus, and took the whole army captive. It would be another generation before the Romans would try to invade Iran again, and when they did, this time under the brilliant general Marc Anthony, they again suffered a catastrophic defeat.
Carrhae was a turning point for Iranian fortunes. Although the Parthians only numbered ten thousand soldiers to Crassus' forty thousand, their combination of range, mobility, and shock tactics nullified the Romans' numerical advantage. Tactics like these enabled the Parthians to create a sizeable empire, stretching across much of former Achaemenid and Seleucid territory.