The Jugurthine War (Latin: Bellum Iugurthinum; 111–105 BC) was an armed conflict between the Roman Republic and King Jugurtha of Numidia, a kingdom on the north African coast approximating to modern Algeria. Jugurtha was the nephew and adopted son of Micipsa, king of Numidia, whom he succeeded to the throne; he had done so by overcoming his rivals through assassination, war, and bribery.

Following Jugurtha's usurpation of the throne of Numidia, a loyal ally of Rome since the Punic Wars, Rome felt compelled to intervene. The war constituted an important phase in the Roman subjugation of Northern Africa and the rise of the empire, but Numidia did not become a Roman province until 46 BC.

Numidia was a kingdom located in North Africa (roughly corresponding to northern modern day Algeria) adjacent to what had been Rome's arch enemy, Carthage. King Masinissa, who was a steadfast ally of Rome in the Third Punic War, died in 149 BC, and was succeeded by his son Micipsa, who ruled 149–118 BC. At the time of his death, Micipsa had three potential heirs: his two sons, Adherbal and Hiempsal I, and an illegitimate nephew, Jugurtha. Jugurtha had fought under Scipio Aemilianus at the siege of Numantia, where, through friendship with Roman aristocrats, he had formed an acquaintance with Roman manners and military tactics. Micipsa, worried that at his death Jugurtha would usurp the kingdom from his own somewhat less able sons, adopted him, and bequeathed the kingship jointly to his two sons and Jugurtha, with the realm to be divided into three. After King Micipsa's death the three kings fell out, and ultimately agreed among themselves to divide their inheritance into three separate kingdoms; however, they were unable to agree on the terms of division, and Jugurtha declared open war on the other two kings. Hiempsal, who, though the younger, was the braver of the brothers, was assassinated by Jugurtha's agents, and Adherbal, unable to defend himself, was defeated and forced to flee to Rome, where he appealed for arbitration to the Roman Senate.

Although the Senate was given authority by Micipsa to arbitrate his will, they now allowed themselves to be bribed by Jugurtha into overlooking his crimes. The Roman Senate organized a commission, led by the ex-Consul Lucius Opimius, to fairly divide Numidia between the remaining two contestants, starting in 116 BC. However, Jugurtha bribed the Roman officials in the commission into allotting him the better, more fertile and populous western half of Numidia, while Adherbal received the east. Powerless against Roman corruption, Adherbal accepted and peace was made. Shortly thereafter, in 113 BC, Jugurtha again declared war on his cousin anyway, and defeated him, forcing him to retreat into Cirta, Adherbal's capital. Jugurtha's goal was now to conquer, and thus unite, Numidia under his rule.

Rome, seeing this as a threat to stability in the region, sided with Adherbal. With Roman help, Adherbal held out for some months, aided by a large number of Roman Equites who had settled in Africa for commercial purposes. From inside his siege lines, Adherbal appealed again to Rome, and the Senate granted his request. The Roman Senate dispatched a message to Jugurtha to end the war and recognize Abherbal's right to his kingdom. Jugurtha ignored the demand, and the Senate sent a second delegation, this time headed by Consul Marcus Scaurus, a respected member of the aristocracy, to threaten Jugurtha into submission. King Jugurtha, pretending to be open to discussion, protracted negotiations with Scaurus and Adherbal long enough for Cirta to run out of provisions and hope of relief. When Scaurus left without having forced Jugurtha to a commitment, Adherbal surrendered. Jugurtha promptly had him executed, along with all Romans who had joined in the defence of Cirta. The deaths of Roman citizens caused an immediate furor among the commoners at home, and the Senate, threatened by the popular tribune Gaius Memmius, finally declared war on Jugurtha in 111 BC and joined the late Adherbal's side in the war, though with reluctance.

Lucius Calpurnius Bestia, consul for the year, was appointed to command the Roman army in Africa against Jugurtha. He was accompanied by Scaurus and other experienced officers, and received an offer of alliance from Bocchus I, king of Mauretania. The defection of Bocchus, his own father-in-law, filled Jugurtha with alarm, and he sent to the Roman consul to surrender. Ceasing to regard him as a threat, the Roman senators granted him a treaty on extremely lenient terms: Numidia was restored to Jugurtha intact, in exchange for bribes to the Senate, a small additional fine, and the remittal of his war-elephants (which he later bought back at reduced price from corrupt officers). In fact, the treaty of Jugurtha's terms of surrender was so favorable that it led to a renewal of the popular outcry at Rome; at the demand of the tribune Memmius, an investigation was launched into the proceedings of the treaty. Jugurtha was summoned to Rome– with the promise of a safe conduct transit –and appeared as a witness. Rather than complying with the inquisition, he bribed two Roman Tribunes to veto the proceedings and prevent him from testifying. In the ensuing outrage, Jugurtha's cousin Massiva, who had fled to Rome in fear of his cousin, seized the opportunity to press his own claim to the Numidian throne. Jugurtha assassinated him, and the Senate, though initially inclined to accept bribery again to allow him to escape retribution, was ultimately compelled by his insolence and by the fury of the mob to expel him from the city and revoke the recent peace. The armies of Rome were to return to Jugurtha's Numidia.

The consul Spurius Postumius Albinus took command of the Roman army in Africa (110 BC), but failed to carry out energetic action, due to incompetence, indiscipline in his army, and – it was alleged – bribery by Jugurtha. Later in the year Albinus returned to Italy, leaving the command to his brother, Aulus Postumius Albinus. The latter, more active though no more able than his brother, decided on a bold strike, marching in mid-winter to besiege the town of Suthul, where the Numidian treasury was kept; however, the town was strongly garrisoned and excellently fortified and could not be captured. Postumius, anxious not to have retreated without striking the enemy a blow, allowed Jugurtha to lure him into the desolate wilds of the Sahara, where the cunning Numidian king, who had reportedly bribed Roman officers to facilitate his attack, was able to catch the Romans at a disadvantage. Half the Roman army was killed, and the survivors were forced to pass under a yoke in a disgraceful symbolism of surrender. The beaten Postumius signed a treaty permanently naming Jugurtha the king of all of Numidia and returning to the peace concluded with Bestia and Scaurus. The Senate, however, when it heard of this capitulation, refused to honour the conditions and continued the war.

After Postumius' defeat, the Senate finally shook itself from its lethargy, appointing as commander in Africa the plebeian Quintus Metellus, who had a reputation for integrity and courage. Metellus proved the soundness of his judgement by selecting men as officers for the campaign based on ability rather than of rank. Both Gaius Marius (a plebeian from Arpinum) and the noted disciplinarian and military theorist Publius Rutilius Rufus documented Metellus' commitment to merit and expertise in his officer corps. ^{[clarification needed]}