The battle of the Muthul River (109 BC) was the first significant Roman victory during the Jugurthine War, but had little long term impact on the course of the war.
The first two Roman commanders in Numidia, Lucius Bestia Calpurnius in 111 BC and Spurius Postumius Albinus in 110 BC, had failed to achieve anything significant, after Jugurtha refused to be drawn into battle. However Spurius soon had to return to Rome to deal with disrupted elections, leaving his brother Aulus in command. Early in 109 BC Aulus decided to attack Jugurtha's treasury at Suthul, but he was drawn into a trap, his army forced out of its night camp after an ambush and he was forced to accept harsh terms to save his army. Unsurprisingly this treaty was rejected by the Senate, and the humiliation also changed the nature of the war, which went from being a campaign to punish a misbehaving ally into a war against a dangerous enemy.
The elections for 109 BC were finally held early in the year. The Numidian command was taken by Quintus Caecilius Metellus, a member of the important Metellus family, who already had an impressive reputation. He raised a new army in Italy, and then moved to Africa to take command of the existing army. Spurius Postumius Albinus had remained in command until then, but he had made no efforts to restoring the discipline of the army, so Metellus had to spend some of his limited time restoring order.
Once the army was ready to move, Metellus invaded Numidia. Jugurtha had quickly realised that he was in trouble, and so once again sent envoys with surrender terms. He offered to surrender everything to the Romans in return for guarantees of safety for himself and his children. He also ordered his officers to provide the Romans with supplies if they invaded. However Metellus either didn't trust Jugurtha's offers, or wasn't interested in them. Instead he attempted to turn Jugurtha's envoys against their master, in the hope that at least one of them would be willing to betray their master. At the same time he sent back encouraging messages, as if he was considering accepting Jugurtha's offer.
Metellus put some thought into how to advance into Numidian territory. He led the way with a force of lightly armed cohorts, archers and slingers. Caius Marius, his second in command and commander of the cavalry was at the rear of the army. The auxiliary cavalry and light infantry were posted on the flanks to protect against any Numidian attack. The heavy infantry of the legions marched in the centre, protected on all sides by light troops. He seized the important trading city of Vaga (modern Beja), and continued to attempt to subvert Jugurtha's envoys.
After seizing Vaga, Metellus probably moved south, crossing the Medjerda valley and the mountains to its south, perhaps heading for the important city of Sicca (modern El Kef). Jugurtha realised that Metellus had no intention of accepting his terms, and decided to fight. He discovered Metallus's route and managed to get his own army head of the Romans, ready to ambush them in the Muthul valley. This was described by Sallust as flowing from south to north, with a range of mountains running parallel from it, about twenty miles away, with a lower hill stretching out from the mountains towards the river. The context of the battle would place these mountains on the west bank of the river. It isn't entirely clear where this river is, but one candidate is the modern Mellag or Mellegue, which flows north a few miles to the west of El Kef.
Metellus's route didn’t follow the river valley. Instead he crossed the mountains, and then had to cross the twenty mile wide plain to reach the river. This placed the lower ridge to his south, on his right if he advanced directly towards the river. Jugurtha spread his army out along this ridge. A commander called Bomilcar was give the elephants and part of the infantry and placed at the river end of the ridge. Jugurtha commanded the cavalry and the best of the infantry, and positioned them on the left, close to the mountains. The rest of the army was probably spread out along the ridge.
As Metellus descended from the mountains from the plains, heading for the river, his right flank was thus closest to the Numidian lines. While he was still in the mountains, Metellus spotted some of Jugurtha's troops on the ridge, but the broken ground and scattered trees meant he couldn't be sure exactly what he was seeing. However he did alter his formation to cope with the threat, organising his right wing into three lines, posting all of the cavalry on the flanks and distributing the slingers and archers amongst the infantry. He then resumed his march down onto the plains, heading towards the low hill instead of towards the river, and with the original right wing as the new front line and the original front as one flank.
At first Jugurtha remained on his ridge. Metellus became worried about his lack of access to water, and decided to send the lightly armed cohorts and part of the cavalry towards the river under the command of Rutilius Rufus. The main army then resumed its advance, now with the original left flank in the lead, under Metellus's direct command, Marius in command in the centre and the old right wing facing the hills.
As soon as the Romans left the mountains, Jugurtha moved 2,000 of his infantry onto their old route to prevent them from retreating in that direction. He then launched his general attack. This came close to causing a disaster for the Romans, who struggled to cope with the Numidian cavalry, which rarely got within close combat range unless they outnumbered a group of pursuing Romans. However the Numidians were unable to force the Romans into flight, and the battle dragged on for some time.
Eventually Metellus was able to regain control over part of his infantry. He managed to gather four cohorts of infantry, and sent then to attack up the ridge towards Jugurtha's infantry, which was resting on top of the ridge. When the Romans reached the top of the ridge the Numidians were forced to retreat, and with the key to his position lost, Jugurtha decided to retreat with most of his army intact.
A separate battle took place nearer to the river. Rutilius reached the river, watched by Bomilcar, who then decided to attack in order to prevent Rutilius moving back to help Metellus. The Romans were alerted to the danger they were in by the dust cloud created by the advancing elephants, and formed up outside their camps. A battle began, but the Numidian elephants soon became caught up in the underground or surrounded by Romans. At this point the rest of the Numidian force broke and fled, leaving forty of their elephants to be killed by the Romans (another four were captured).
The fighting was now over on both fronts. Metellus began to march towards the river, while Rutilius marched back towards the ridge, worried about how long it was taking Metellus to reach the river. The two Roman forces almost clashed in the dark, but this was just about avoided.
In the aftermath of the battle Metellus spent four days in the camp by the river, allowing his men to rest and the wounded to recover. Although Jugurtha had escaped, most of his army had dispersed, although his own royal cavalry remained loyal. He thus had to spend this time raising a new army.
After resting for four day Metellus advanced into the richest part of Numidia, laying waste to it as he advanced. The Roman army was split into two columns, with Metellus commanding one and Marius the other. Jugurtha harassed each column in turn, but refused to risk another battle. Eventually Metellus decided to besiege Zama, in the hope that this would force Jugurtha into risking another battle, giving the Romans another chance to capture him.