The battle near Utica (81 BC) was a victory for a Sullan army led by the young Pompey over a Marian army led by Gnaeus Domitius Ahenobarbus.
During Sulla's first civil war of 88-87 BC Ahenobarbus had sided with Marius. His actions during Sulla's second civil war of 83-82 BC are unknown, but after Sulla seized power he fled to Africa. During the first civil war Marius had fled to Africa, and had been able to raise an army which he led back to Italy. Ahenobarbus was also able to raise a powerful force, with the aid of the rather obscure Hiarbas, king of Numidia. Hiarbas appears to have usurped the throne of Hiempsal II, the king when Marius fled to Africa, possibly with the aid of Ahenobarbus. Between them Ahenobarbus and Hiempsal were able to raise an army 27,000 strong.
In 81 BC the young Pompey Magnus, who had sided with Sulla in the civil war, was sent to Sicily where he successfully deposed the Marian governor. While still on Sicily he received orders from Sulla and the Senate to go to Africa to attack Ahenobarbus.
Plutarch gives us the longest account of the African campaign. Pompey raised a fleet of 120 galleys and 800 transports, which he split in two. Half landed at Utica and half at Carthage, within the Roman province. Soon after Pompey landed seven thousand of Ahenobarbus's men deserted to him. He had probably crossed over with six complete legions, around 30,000 men, so this raised his force to 37,000 men, while Ahenobarbus was reduced to 20,000.
The deserters appear to have been the only Marian troops close to Pompey at this stage, as for the next few days he lost control of his army. Some of his soldiers found hidden money. The rest of the army decided that this must be part of a larger Carthaginian treasure, possibly buried during the Third Punic War. For the next few days Pompey's men turned into treasure hunters, and his only option was to led the treasure fever burn itself out.
By the time Ahenobarbus reached the area, Pompey had his troops back under control. Ahenobarbus formed up his army behind a ravine or wadi, but that morning a violent storm hit. After a bit Ahenobarbus decided that the wind and rain meant that there would be no battle that day, and ordered his men to return to their camp.
Pompey decided to take advantage of this movement, and ordered his men to attack across the ravine. The storm caused some problems for his own troops, who could hardly see each other, and Pompey was nearly killed after he failed to give the countersign quickly enough. Ahenobarbus's forces were caught out of formation. Not all of his men attempted to fight, and those that did struggled to get back into formation. Only 3,000 of Ahenobarbus's 20,000 escaped from the battlefield. Ahenobarbus was amongst those who managed to escape back to his camp.
In the aftermath of the battle Pompey's men hailed him as imperator, but he refused to accept the honour until Ahenobarbus's camp had been taken. His men made an immediate attack on the camp, which quickly fell. Ahenobarbus was killed in the fighting at the camp.
In the aftermath of this victory Pompey invaded Numidia. King Hiarbas also faced an invasion from the west, led by King Bogudes of Mauritania, whose father had been an ally of Sulla. Hiarbas was captured, and Hiempsal restored to his throne.
Orosius, writing in the 5th century AD, produced a much shorter account of the campaign. He reports that Pompey killed eighteen thousand of his opponents, similar to Plutarch's figure of seventeen thousand casualties, but states that Ahenobarbus was killed in the battle, fighting in his vanguard. In the aftermath Hiarbas was captured at the town of Bulla (now in the north-west of Tunisia, then at the eastern edge of Numidia).
By now Sulla appears to have become concerned that Pompey had control of a powerful army, and decided to split it up. He ordered Pompey to send five of his legions back to Italy, and remain in Africa with the sixth until his replacement arrived. Pompey's men refused to accept these orders, and refused to go home without Pompey. Pompey eventually managed to calm them by threatening to commit suicide if they tried to force him to disobey Sulla's orders.
This disorder must have lasted for more time than the single day that Plutarch's account implies, for news reached Sulla that Pompey was in revolt, something that was unlikely if the incident was defused in one day. Sulla is reported to have told his friends that it was 'his fate, now that he was an old man, to have his contests with boys'. When Pompey turned out to have remained loyal, Sulla met him outside Rome, and greeted him as Pompey Magnus, or Pompey the Great. Pompey himself didn't use this name in writing for a few years, only starting after he was sent to Spain to take command of the war against Sertorius in 76 BC.
On his return to Rome Pompey insisted that he should be allowed to celebrate a triumph, having defeated a foreign foe in Africa, despite only being twenty four and not yet a senator. At first Sulla refused, but Pompey was determined to get his way and warned Sulla that 'more worshipped the rising than the setting sun'. Sulla gave in with good grace, and allowed the triumph. Not everything went Pompey's way. Some of his men threatened to interrupt the triumph because they didn't think they'd been paid enough, but Pompey refused to give way. He was unable to overcome the narrow gates of Rome, which weren't big enough for the elephants he had brought back from Africa, but this didn't spoil his day, and he became the first Roman knight to celebrate a triumph.