The Battle of Ad Decimum took place on September 13, 533 between the armies of the Vandals, commanded by King Gelimer and the eastern Roman Empire, under the command of general Belisarius. This event combined with events in the following year are sometimes jointly called the Second Battle of Carthage. The victory marked the beginning of the end for the Vandals and began the "Reconquest" of the west under the Emperor Justinian I.
Ad Decimum (Latin for Ten Mile Post), was simply a marker along the Mediterranean coast 10 miles south of Carthage. Gelimer, with 11,000 men under his command, had advance warning of the approach of Belisarius' 17,000-man army and chose to take a strong position along the road to Carthage near the post marker. He divided his forces, sending 2,000 men under his nephew Gibamund across a salt pan in an effort to flank Belisarius' army, which was advancing in narrow columns along the road. Another Vandal force, under Gelimer's brother Ammatas, was assigned to initiate a holding action at a defile near Ad Decimium. If everything worked well, Gelimer's 7,000-man main body would follow Gibamund around the Roman left flank and cut off their retreat.
Gibamund failed to accomplish his mission, as a force of Romans and Hun mercenaries drove his 2,000-man force off and killed him. Ammatas also failed; he arrived at the defile with his men still strung out along the road back to Carthage, and he too was killed. The Romans pursued his men all the way to the gates of Carthage itself.
On the other hand, Gelimer's main body was getting much the better of Belisarius' forces along the main road. Belisarius' mercenary cavalry was routed by the Vandals, and even though Gelimer was outnumbered, his men were doing much better in the fighting. It appeared as though the Vandals would win the battle.
But when Gelimer reached Ammatas' position and discovered that his brother had been killed, he became disconsolate and failed to give an order for one more assault – which likely would have destroyed the reeling Roman army and cut off the Huns and Romans who had earlier advanced toward Carthage after beating Ammatas and Gibamund. Instead, his men let their guard down while Gelimer buried his brother on the battlefield.
Given a respite, Belisarius was able to regroup his forces south of Ad Decimium and launch a counterattack, which drove the Vandals back and soon routed them. Gelimer was forced to abandon Carthage.
Belisarius camped near the site of the battle, not wanting to be too close to the city at night. The next day he marched on the city, ordering his men not to kill or enslave (as was normal practice at the time) because the people were actually Roman citizens under Vandal rule. They found the gates to the city opened, and the army was generally welcomed. Belisarius went straight to the palace and sat on the throne of the Vandal King. He then set about rebuilding the fortifications of the city, and his fleet sought shelter in the lake of Tunis, five miles south of Carthage.
After a second defeat at the Battle of Ticameron later in the year, the Vandal kingdom was all but ended.