At Ad Decimum on 13 September 533, Gelimer's plan to trap the Roman column ten miles from Carthage collapsed on timing, and Belisarius won the road to the city.
Ad Decimum — "at the tenth milestone" — was fought on 13 September 533 on the road from the landing beaches to Carthage.
Gelimer's plan was genuinely good: three forces converging simultaneously on a strung-out marching column, with his own troops closing the trap from behind.
It failed entirely on timing, and the battle has become the standard illustration of what happens when a converging attack loses coordination.
Belisarius had landed unopposed some days earlier and was marching north with his army in a long column, his cavalry screening ahead and the fleet shadowing the coast.
Gelimer was behind him rather than in front, and saw the opportunity to catch the column between three forces at a defile where the road narrowed.
His brother Ammatas was to come out from Carthage and block the head of the column, his nephew Gibamund to strike the flank across the plain, and Gelimer himself to fall on the rear.
Ammatas arrived early and with only part of his force, ran into the Roman advance guard, and was killed. His troops fled back toward Carthage, colliding with their own reinforcements coming the other way.
Gibamund's flanking force was met by the Hunnic cavalry of the Roman army on the open plain and destroyed before it reached the column at all.
Gelimer arrived last, drove back the Roman cavalry and for a moment held the field. Then he came upon his brother Ammatas's body, stopped to mourn and to arrange the burial, and let the initiative go. Belisarius rallied his cavalry, attacked, and the Vandal army broke.
The road to Carthage was open. Belisarius entered the city on 15 September and, in a scene Procopius clearly enjoyed, dined that evening in the Vandal royal palace on the meal prepared for Gelimer.
He kept his army under strict discipline and paid for supplies, which kept the African population neutral — a decision that mattered more than the battle itself for what followed.
Gelimer withdrew inland to gather what was left, and recalled his brother Tzazon and the Sardinian expedition.
The battle decided the campaign within weeks of the landing. A kingdom of a century fell because its field army was beaten in an afternoon on a road outside its capital.
It is also the best surviving illustration of the coordination problem in pre-modern warfare. Three forces without communication cannot converge on a moving target, and Gelimer's plan required exactly that.
The detail about the mourning is Procopius, and it should be treated with some care — a defeated enemy undone by brotherly grief is a very tidy story. But it is the account we have, from a man who was there.