The Battle of Naissus took place in September of 269 between the armies of the Goths and forces of the Roman Empire, led by either Gallenius or Claudius II as emperor and the future Emperor Aurelian as cavalry commander.
The battle came about as a result of a massive invasion of the Goths into Roman territory in late 268 and in the early months of 269. The Goths pushed across the Danube River and made their way into the wealthy Roman province of Pannonia, where they looted and sacked several cities. It was thought by many that their next stop was going to be Rome itself.
Gallenius checked the Goths by winning an impressive victory in the spring, probably in April, but the Romans were weakened by decades of internal strife and rebellions, and were unable to expel the Goths from their territory. The Goths continued their depredations throughout the summer, until Gallenius led a second expedition against them as the fall began.
There is some dispute about who commanded the Roman army in the ensuing battle, as Gallenius died at about the same time and Claudius, known to history as Claudius Gothicus, later was given credit for the victory by the Roman Senate. Gallenius was almost certainly alive and present at the battle, so credit for the victory as emperor is his, but it appears that Claudius and especially Aurelian did most of the fighting.
In a bitterly contested, no-quarter showdown near Naissus (the modern Nis, Serbia and Montenegro), it was Aurelian who decided the battle in the Romans' favor when his cavalry routed the vaunted Goth heavy cavalry and then stormed the Gothic laager. In the ensuing chaos, between 30,000 and 50,000 Goths were killed or wounded, and thousands more taken prisoner. Many of the prisoners later chose to join the Roman army and served in the later, victorious campaigns of both Claudius and Aurelian.
More importantly, the devastating defeat, coupled with the earlier defeat in April of the same year, broke the power of the Goths. Some remained on Roman soil until 271, when Aurelian drove the last of them back across the Danube, but they were no longer a danger to Rome or any other vital Roman area. In fact, a century would pass before the Goths would again seriously threaten the empire.