The Battle of Carrhae in 53 B.C. was one of the biggest military disasters Rome ever suffered, ranking right up there with Cannae, The Teutoberg Forest, and Lake Trasimene. The battle occurred in what is today Syria between a Roman army under Marcus Licinius Crassus and a Parthian (Persian) army under a general Surena. In the battle, seven legions were destroyed and their Eagles taken and Rome did not trouble the Parthian Empire again for almost 50 years.
The battle was written about by both Livy and Plutarch. The links are to translations of their texts.
The battle had its origins in the glory seeking of Crassus himself. He was part of the First Triumvirate with Pompey and Caesar and was assigned as the governor of the Roman province of Syria, which bordered the Parthian empire. He thought to gain glory for himself by conquering Parthian territory and adding it to Rome. To that end, he used the seven Legions with 40,000 men under his command and began to march on Parthian territory.
In May of 53, Crassus marched into Parthian territory and met the Parthian army of Surena on a large plain near the town of Carrhae. Crassus drew his army up into battle array with great confusion. According to Plutarch, at first he spread the army out to prevent its being flanked but then closed it up right before the battle began. The Romans had a mainly infantry army as was typical but the Parthian army was organized along lines that the Romans had never seen before. Almost the entire Parthian army was mounted and it was mostly light cavalry archers with a small contingent of heavy cavalry Cataphracts.
The Parthians also fought differently than the Romans, they did not close and engage in melee combat as was the Roman norm, instead the archers would ride close, but still out of reach of the Roman infantry and loose a cloud of arrows before riding out of range again. That tactic is the origin of the contemporary saying about the “Parthian Shot. The Romans however, fought in the modified Marian phalanx that, seen from above, resembles a checkerboard. They engaged in melee combat. A typical Roman battle began with the infantry throwing their Pilum, a short articulated spear, and then charging to close with shield and short sword.
The Parthians attacked the vanguard of the Roman force and virtually annihilated it whereupon Crassus retreated to the village of Carrhae. The Parthians approached and asked him to meet with their commander to talk terms. When Crassus reached the Parthian camp he was taken, bound, and decapitated.
After the death of Crassus, the Roman army scattered in a bid to escape. The vast majority were hunted down and killed. Only about 5,000 of the original 40,000-man force eventually escaped. The worst thing from the Roman point of view is that the standards of the seven legions were lost to the Parthians; a huge stain on Roman honor. They were not returned for 30 years when Augustus, the first Emperor, negotiated their return.
In the end, Carrhae was an unmitigated disaster for the Romans. It did two things mostly. One, it showed that the Romans were not invincible and two, it further destabilized the Roman world because the loss of Crassus removed a stabilizing influence on the First Triumvirate. I will not go so far as to say that civil war would have been avoided if Crassus had not been defeated. However, I think it is safe to say that perhaps civil war would have been delayed for some time if he had not been killed. The contrary view is that Crassus was an idiot and deserved his fate and that conflict between Pompey and Caesar was inevitable anyway and the elimination of Crassus just clarified the lines of the coming contest.