The battle of Sacriportus (82 BC) was a key battle of Sulla's Second Civil War, and saw him defeat the army of the consul Marius the Younger. In the aftermath Marius was besieged in Praeneste, while Sulla was able to occupy Rome without a fight.
At the start of the campaign of 82 BC both sides split their armies. Sulla sent Metellus Pius and Pompey to campaign in the north, where the Marians had strong support in Cisalpine Gaul. Sulla himself advanced north from Campania to attack Rome.
The consuls for 82 BC also split up, with Gnaeus Papirius Carbo heading north, while Marius the Younger, son of the great general Gaius Marius, moved south to deal with Sulla.
Very few details of Sulla's advance north have survived, but he must have encountered some resistance. We first encounter him attacking Setia (Sezze), a town on the edge of the mountains that bordered the Pontine Marshes, about 40 miles south of Rome. Marius advanced south to try and save the town, and was camped nearby when it fell. He retreated north, across the hills, towards Signia (modern Segni), 13 miles to the north. Sulla pursued him, and the two sides clashed at Sacriportus. Unfortunately we don’t know exactly where this was, but from the context it was probably somewhere between Signia and Praeneste, another 12 miles to the north-west of Signia.
Plutarch provides the most details of the battle. He locates it at Signia, and gives Marius eighty-five cohorts (just over 40,000 men). Sulla had a dream in which he saw the older Marius warn his son not to fight on the following day, and was determined to fight on that day. However his army was split in two, with a section under Dolabella (father of the Dolabella of Caesar's era). Sulla tried to summon Dolabella to join him, but Marius blocked the roads. Sulla's men attempted to fight their way through, but a combination of fierce resistance and heavy rain wore them out. Sulla's military tribunes came to him and asked him to postpone the battle to the following day, as the troops were exhausted. Sulla reluctantly agreed, and ordered his men to pitch camp.
Marius attempted to take advantage of this by attacking while Sulla's men were digging the trench outside the camp. Although he wasn't as successful as his father, the younger Marius didn't lack courage, and on this occasion Plutarch has him leading from the front (Plutarch does also mention that some writers claimed that he actually slept through the battle. Sulla was angered by this attack, and his troops picked up his anger and used it to inspire their resistance. After a brief period of close quarters fighting, Marius's men broke and fled, suffering heavy casualties.
Appian provides very few details of the course of the battle, but does add to our knowledge of its end. Marius's left wing began to give way. Five cohorts of infantry and two cohorts of cavalry decided to desert to Sulla, and this triggered a more general collapse of Marius's line.
Both sources agree that Marius fled to Praeneste. The first refuges were let into the city, but the gates were closed before Marius arrived, and he had to be hauled up the walls on a rope (or in a basket).
Plutarch reports that Sulla claimed to have killed 20,000 of Marius's men and taken 8,000 prisoners, while only losing 23 men himself. Appian reports that there was more fighting around the walls of Praeneste, in which more of Marius's men were killed. Amongst the prisoners were a large number of Samnites, who had supported the Marian cause since Sulla's First Civil War (88-87 BC). Sulla had all of the Samnites killed, something that he would repeat after the battle of the Colline Gate later in the year.
The siege of Praeneste lasted for the rest of the war. Carbo made several attempts to lift the siege, each of which failed. His Samnite allies also attempted to break through Sulla's siege lines, again without success. Towards the end of the war the main Marian leaders fled into exile, and the Samnites decided to abandon their attempts to help Marius and instead attacked Rome. They were defeated after heavy fighting outside the Colline Gate, and in the aftermath of this defeat the defenders of Praeneste finally gave up. Marius committed suicide before he could be captured.