The siege of Cirta (112 BC) saw Jugurtha win a brief Numidian civil war, but his actions after the fall of the city provoked a Roman intervention, which eventually ended his reign (Jugurthine War)
In 118 BC King Micipsa of Numidia died, leaving the kingdom to his sons Hiempsal and Adherbal, and his adopted son Jugurtha. Unsurprisingly this arrangement quickly failed. Hiempsal was almost immediately murdered by one of Jugurtha's supporters, and Adherbal was forced to flee to Rome after a short civil war. He appealed to the Senate, which appointed commissioners to decide the issue. The commissioners divided Numidia in half. Jugurtha got the western half, which Sallust described as the most fertile and populous, while Adherbal got the eastern half, which had more towns and cities, and included the capital at Cirta and the areas taken from Carthage before and after the Third Punic War.
Jugurtha waited until the commissioners had left Africa before he broke the agreement. His first move was to carry out a raid into Adherbal's half of the kingdom, hoping to provoke Adherbal into a retaliation that would give him an excuse for war. Adherbal refused to rise to the bait, aware that he had quickly been overwhelmed in the first war. He sent ambassadors to Jugurtha to complain about the raid, but unsurprisingly they returned without satisfaction.
Jugurtha followed his raids with a full scale invasion. Adherbal was finally forced to raise an army to try and defend his kingdom. The two armies met up close to Cirta, but it was late in the day, so both prepared to camp. Once Adherbal's army was encamped, Jugurtha launched a surprise attack on the camp. Adherbal's army was routed, while Adherbal himself only just escaped to Cirta. Jugurtha's men were close behind, but they were repulsed by some of the Italians in the town. Sallust described them as 'Togatorum' when they are first mentioned, 'toga wearers', and as Italians later on, so it isn't clear if there were any full Roman citizens present.
Jugurtha had failed in his immediate objective, which had been to win the war quickly, before the Romans could intervene. He now began a full scale siege of Cirta, deploying a range of siege engines. Adherbal had sent messengers to Rome before the battle outside Cirta, and as he had expected the Senate attempted to intervene. However their first attempt was ineffective - three young men were sent to Africa, with orders to tell both Jugurtha and Adherbal to lay down their arms and settle their differences by arbitration. News of the battle arrived while this first deputation was still in Italy, speeding up their departure. Jugurtha received the envoys, but refused to lift the siege, or to allow the deputation to visit Adherbal.
It appears that Jugurtha had suspended active operations while the deputation was in Africa, but after they left he renewed his efforts. He surrounded Cirta with a ditch and rampart, protected with towers, and made a series of attempts to capture the city, some by force and some by bribing or threatening the defenders.
Adherbal was able to get another message through the siege lines and to Rome. By this point the siege had been going on for five months, and it lasted long enough for a second, much more senior, Roman delegation to reach Cirta. Amongst the ambassadors was Marcus Scaurus, a former consul and 'princeps senatus'. At first Jugurtha attempted to finish the siege before the ambassadors could intervene, but their increasingly urgent messages eventually forced him to visit them in the Roman province. Even then the ambassadors were unable to make any impact on Jugurtha, and returned to Rome without achieving anything.
When the news of this failure reached Cirta, Adherbal's Italian supporters lost heart, and insisted that he should surrender the city to Jugurtha. Adherbal was forced to agree, and surrendered on terms - his life was to be spared, and all other matters would be settled by the Roman senate. The Italians believed that their own safety was guaranteed by the active Roman interest in the siege.
Jugurtha now made a fatal mistake. He had Adherbal tortured to death, which perhaps wasn't a surprise, but he also ordered a massacre of all of the Numidian and Italian occupants of Cirta. Even Jugurtha's supporters in Rome weren't able to prevent a declaration of war, and the task of punishing Jugurtha was given one of the newly elected consuls for 111 BC, Lucius Bestia Calpurnius.