The Battle of Carrhae

But the Parthians now stood at long intervals from one another and began to shoot their arrows from all sides at once, not with any accurate aim (for the formation of the Romans would not suffer an archer to miss his man even if he wished it), but making vigorous and powerful shots from bows which were large and mighty and curved so as to discharge their missiles with great force.

Plutarch, Life of Crassus, 24.5 (translated by Bernadotte Perrin)

The battle of Carrhae - modern-day Harran in Turkey - in May 53 BC, was one of the most significant conflicts of the Roman-Persian wars. A force of more than 40,000 Romans, led by the triumvir Marcus Licinius Crassus, met their match in the formidable Parthian mounted archers. Three-quarters of the Romans were killed or captured, including Crassus himself, and the Roman advance into Persian territory was halted.

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:

The Parthians stationed their mail-clad horsemen in front of the Romans, and then with the rest of their cavalry in loose array rode round them, tearing up the surface of the ground, and raising from the depths great heaps of sand which fell in limitless showers of dust. . . Then, as the enemy got to work, their light cavalry rode round on the flanks of the Romans and shot them with arrows, while the mail-clad horsemen in front, plying their long spears, kept driving them together into a narrow space, except those who, to escape death from the arrows, made bold to rush desperately upon their foes. These did little damage, but met with a speedy death from great and fatal wounds, since the spear which the Parthians thrust into the horses was heavy with steel, and often had impetus enough to pierce through two men at once.

Plutarch, Life of Crassus, 25.7, 27.1

Etching of cataphract on rock, Sasanian era, British Museum

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.