The siege of Cirta (January 46 BC) saw Caesar’s allies capture and sack one of the key cities of Numidia, forcing King Juba to withdraw most of his troops from the Republican army, weakening it just as Caesar was at his most vulnerable (Great Roman Civil War).
In the aftermath of their defeat at Pharsalus many of the remaining Republican leaders fled to North Africa. Much to their relief Caesar became entangled in Egypt, giving them a year and a half to raise new armies and prepare to meet his inevitable invasion. They also formed an alliance with King Juba I of Numidia, whose kingdom almost surrounded the Roman province. He had four legions, along with large numbers of cavalry, light infantry and archers, and was a potent ally.
Caesar crossed to Africa at the start of 46 BC, but his army was scattered at sea, and for some time he was dangerously isolated on the east coast of the province, facing a much larger Republican army under Metellus Scipio. A standoff soon developed, and Juba decided to bring his army into the Province, to support Scipio. This would have left Caesar even more outnumbered, and very vulnerable to an early defeat.
Luckily for Caesar, he also had allies in Africa, in the shape of Bogud, the brother of the king of Mauritania. Bogud in turn was supported by P. Sitius, a Roman adventurer. When Juba led his army into the Province, Bogud and Sitius invaded Numidia and besieged Cirta, the wealthiest city in the country.
After arriving outside Cirta, Bogud and Sitius offered the inhabitants fairly generous terms, allowing them to leave safely if they would surrender the city. These turns were rejected, and the city was then stormed and the citizens all killed.
When this news reached Juba, he decided to withdraw back into his kingdom with most of his troops. He left thirty elephants with Scipio, and returned home to try and expel Bogud and Sitius. This left Scipio without his support when Caesar was at his most vulnerable. Juba returned towards the end of the war, but appears not to have played a major role in the final battle at Thapsus.