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.