What are the most one-sided battles in human history?

The Battle of Carrhae (6 May 53 BC)

This was a decisive battle during the war between the Roman Republic and Parthian Empire, where an invading force of seven Roman legions (around 43,000 men) under the command of Marcus Licinius Crassus were massacred by the Parthian light horse archers and heavy cataphracts.

Roman troops under the attacks of Parthian horse archers.

Historically, the Battle of Carrhae was also one of the most disastrous defeats throughout the Roman history, in which “20,000 Romans were killed, 10,000 were captured, and only about 5,000 escaped the carnage.”

At the beginning of the battle, the Parthian horse archers applied the special technique which was called the “Parthian shot”in order to shower the Roman Army with deadly arrows, while they were freely moving in front of the pursuing army. The Parthian arrows were indeed very deadly to the Roman troops, as they suffered numerous casualties from the Parthian’s continuous “rains of arrows”.

The attacks of the Parthian heavy cataphracts.

The next phase of the battle came the Parthian heavy cataphracts as they appeared to attack and blocked the Roman retreat. The first day of the battle ended with a total disaster for the Roman Army, with the loss of Publius Licinius Crassus, son of Marcus Licinius Crassus. Then, Crassus came to negotiate with the Parthians, where himself and his generals were soon killed.

Finally, the Battle of Carrhae ended with a catastrophe to the seven Roman legions, as they lost 20,000 men on the battlefield and 10,000 were captured. On the contrary, the Parthian casualties were minimal. Luckily, Gaius Cassius Longinus led approximately 10,000 surviving Roman troops to broke the Parthian lines and successfully retreated back to Syria, where he soon reorganized the defensive system to repel further Parthian attacks.

It could be seen that the Battle of Carrhae was among the most one-sided battles in human history, with the destruction of the whole seven Roman legions while the Parthians only suffered the minimal losses. Once again, thank you very much for reading my answer and best regards,

Andrew