The siege of Piraeus of 87-86 B.C. was a bitterly fought clash that only ended when the defenders of the city pulled out by sea after the fall of the city of Athens. The Pontic involvement in Greece had begun towards the end of 88 B.C., when an army under the general Archelaus was sent to Athens, at the invitation of the anti-Roman party in the city, led by the philosopher and politician Aristion. Large parts of southern Greece rose against the Romans, and for a short period it looked as if Roman authority in Greece would collapse as quickly as it had in the province of Asia at the start of the war. A five legion strong consular army under Lucius Sulla was on its way, but wouldn't leave Italy until the start of 87 B.C. The only Roman forces in the area were two legions in Macedonia, but they were busy fighting the Thracians. A small detachment under Q. Bruttius Sura was sent south, and despite is size managed to disrupt Archelaus's plans. Bruttius held his own in three clashes (or a three day long battle) in Boeotia, and Archelaus pulled back to Athens.
Sulla landed in Greece in the spring of 87 B.C., probably in Aetolia. After gathering reinforcements from Aetolia and Thessaly he advanced east through Boeotia, and soon threatened Athens. By 87 B.C. the Long Walls between Athens and Piraeus were in ruins, and so Archelaus and Aristion had to fight two separate sieges. While Aristion took command of the defence of Athens, Archelaus conducted the defence of Piraeus. He had by far the easier job, for Mithridates still had command of the sea, allowing reinforcements and supplies to reach Archelaus. In contrast Aristion was completely isolated in Athens, and by the spring of 86 B.C. starvation was rife in the city.
The twin sieges began in the autumn of 87 B.C. That year Sulla concentrated his efforts against Piraeus, while a smaller force blockaded Athens. Sulla first attempted to take Piraeus by assault, but this first effort was repulsed. He then pulled back to his main bases at Eleusis and Megara to build siege engines. Sulla had two main problems at this stage. Even at the start of the siege he was outnumbered by the defenders of Piraeus, and Archelaus could hope to receive reinforcements. Sulla in contrast had now fallen dramatically out of favour in Rome, where his enemies had seized the city. In 86 B.C. a new army, under the command of the consul Flaccus, was to be sent to Greece, officially to attack Mithridates, but actually to take on Sulla (this army eventually reached Asia, but only after Flaccus had been deposed by his legate C. Flavius Fimbria).
Sulla's second assault on Piraeus also ended in failure, but only after a prolonged period of bitter fighting. The Romans began by using the remains of the Long Wall to raise mounds to bring their siege engines up against the walls, but Archelaus's engineers dug tunnels under the mounds and collapsed them. Both sides then began to dig tunnels under the walls, and a great deal of fighting took place underground.
Although Sulla's engineers did eventually breach the outer walls of Piraeus, Archelaus was able to build new walls inside the town, which proved to be too difficult to breach, and as winter approached Sulla pulled most of his troops back to Eleusis.
In 86 B.C. a second Pontic army appeared on the scene, under the command of Arcathias, son of Mithridates. This army was advancing through Thrace and Macedonia, and there was a real danger that Sulla would be trapped between the two armies. The pressure began to lift in the spring. On 1 March the Romans broke into Athens, and although Aristion escaped into the Acropolis Sulla was now free to concentrate on Piraeus. At about the same time Arcathias died.
Although the prince was dead, his army was still intact, and the threat from the north probably played a part in motivating Sulla's men in their attacks on Piraeus. Archelaus was forced to pull back onto the Munychia peninsula, where he was protected by the sea on three sides and by strong fortifications on the fourth. He could probably have held out here for as long as Mithridates kept command of the seas, but he now realised that there was no point in prolonging the siege. The Pontic garrison evacuated Piraeus by sea, and joined with the northern army in Thessaly. Sulla left a small force to continue the blockade of the Acropolis, and took the rest of his force into Boeotia, where he soon won two decisive battles, at Chaeronea and Orchomenus, which together ended the war in Greece.