A large Roman army commanded by Marcus Licinius Crassus was defeated by a numerically inferior Parthian force under Surenas near Carrhae in northern Syria. From Rogueclassicism.

The Roman army greatly outnumbered that of the Parthians, so this defeat was quite traumatic to the Romans. Subsequent historians, Roman and modern, have tried to explain it away by saying:

1. Crassus was an incompetent general.
2. He was misled by treacherous locals he had trusted.

The campaign is analyzed in The Defeat of Rome in the East. The author points out that:

1. Crassus was a good general: He defeated Spartacus.
2. Considering his record in the chaotic politics of late republican Rome, there is no evidence that he ever trusted anyone in his adult life.

While Crassus was a good general, his opponent, Surenas, was a great one. He reformed the organization of his army to maximize the advantages of its superior cavalry, and to nullify the those of the Roman infantry on the particular terrain of the battlefield. Hence he was able to win an overwhelming success.

This could have changed the course of history, except that Parthian politics were even more dysfunctional than those of Rome. Jealous of his success, the Parthian King had Surenas murdered. Without such a talented general, the Parthians were unable to exploit their success and Rome was able to maintain a defensive position on its Ostfront.