Archelaus was the most prominent Pontic general during the First Mithridatic War (89-85 B.C.), fought between the forces of Mithridates VI of Pontus and the Roman Republic. Archelaus was the Pontic commander during the invasion of Greece, where he suffered a series of heavy defeats that convinced Mithridates to open peace negotiations.
Archelaus's brother Neoptolemus came to prominence during Mithridates's wars on the northern shores of the Black Sea, but Archelaus is not heard of until the start of the First Mithridatic War in 89 B.C. Mithridates began the war by advancing into Bithynia. The two brothers commanded his advance guard, and won the first victory of the war, on the Amnias River, where they defeated the army of Nicomedes IV of Bithynia. Neoptolemus then went on to defeat a Roman-led army at Protopachium, although probably without Archelaus.
Archelaus is next heard of during the campaign in which Mithridates conquered most of the Roman province of Asia. During this campaign Archelaus attacked the city of Magnesia (either Magnesia ad Sipylum or the nearby Magnesia on the Maeander). The attack failed, and Archelaus was wounded in the fighting.
Mithridates's successes in Asia encouraged the Athenians, partly inspired by the tyrant Aristion, to break with Rome and invite Mithridates to operate in Greece. Mithridates responded in the late summer or autumn of 88 B.C. by sending a fleet and army under the command of Archelaus to Athens. On the way across the Aegean Archelaus captured the Cyclades islands and the sacred treasury at Delos, giving the treasure captured there to Aristion to bolster his support at Athens.
Archelaus landed at Piraeus, and encouraged by the presence of his army large parts of southern Greece rose against the Romans. The Romans had two legions available to defend Greece and Macedonia,, but the governor of Macedonia, C. Sentius, was occupied in a war against the Thracian tribes, and was only able to send a small detachment south, under the command of his legate Q. Bruttius Sura. Five more legions, under the command of the consul Lucius Sulla, were on their way, but would not arrive until the spring of 87 B.C.
Archelaus and Bruttius Sura clashed over three days near Chaeronea, in western Boeotia. The inconclusive fighting ended when Archelaus received reinforcements from Sparta and Achaea, but Archelaus then pulled back to Athens and Piraeus, for Sulla's advance guard, under his quaestor L. Lucullus, had now arrived in Greece.
Archelaus and Aristion now found themselves besieged in Piraeus and Athens respectively. Mithridates still had command of the seas, and so Archelaus received supplies and reinforcements, but Athens was securely blockaded. Archelaus made several attempts to get supplies into the city, but without success. Meanwhile a second, much larger Pontic army, under the command of Mithridates's son Arcathias, was advancing through Thrace and Macedonia. If Archelaus and Aristion could hold out long enough, then Sulla would be trapped between two large armies.
The Pontic plan fell apart in the spring of 86 B.C. On 1 March Sulla finally broke into Athens, and although Aristion escaped onto the Acropolis Sulla was now able to concentrate more of his efforts against Piraeus. At about the same time Arcathias died, having reached as far as Tisaeum in Magnesia.
Archelaus and the defenders of Piraeus were soon pushed back onto the peninsula of Munychia, where they were still sheltered by strong defences, but with Athens lost and the port of Piraeus no longer fully usable Archelaus realised that there was no reason to continue to defend Piraues, and decided to evacuate. He took his army north to join up with the forces in Thessaly, taking command of the combined armies at Thermopylae.
Sulla moved into Boeotia to face Archelaus. He was partly motivated by a need to move away from Attica, which had been exhausted by the long sieges of Athens and Piraeus, and partly because a Roman detachment under Hortensius was in danger of being cut off in Thessaly.
Archelaus now suffered two crushing defeats within a few weeks of each other, which effectively ended the war in Greece. Sulla had taken up a position on the hill of Philoboeotus, at the southern edge of the plain of Elatea, with according to his own account only 15,000 infantry and 1,500 cavalry (some of his army was still besieging the Acropolis at Athens). Archelaus may have had as many as 120,000 men, and he definitely outnumbered the Romans in cavalry. Despite this Archelaus eventually moved south east, away from the plain of Elatea, to threaten Chaeronea. On ground that was much better suited to the Romans than to his army he suffered a crushing defeat, losing all but 10,000 of his men.
Archelaus escaped to Chalcis, where he received 80,000 reinforcements. Sulla still lacked a fleet, and so Archelaus was able to carry out raids around the Greek coast, reaching as far west as Zacynthos and the Adriatic, even destroying some of the transports carrying the advance guard of Sulla's replacement, Flaccus. He was also able to choose the site of the second battle, on the plains of Orchomenus.
This time the battle was fought on ground that was well suited to Archelaus's cavalry, but despite this a second crushing defeat followed. This time Archelaus was forced to hide for two days in the swamps of Copais, before once again managing to escape to Chalcis. This time almost none of his soldiers returned to the colours.
When news of the disaster at Orchomenus reached Mithridates he ordered Archelaus to open peace negotiations with Sulla. The first peace conference took place at Aulis late in 86 B.C. Archelaus began by suggesting that Sulla should recognise the status quo in Asia, and in return Mithridates would ally with him against his enemies in Rome. Sulla responded by inviting Archelaus to surrender his fleet and change sides.
Sulla's final terms were that Mithridates should surrender part of his fleet, and all conquered territory in Asia, including Paphlagonia and Cappadocia and pay an indemnity of 2,000 talents. In return for this he would become a friend and ally of Rome. Archelaus remained with Sulla while these terms were sent to Mithridates. By now he was beginning to worry about the reception he would receive in Pontus, and was also growing closer to Sulla, accompanying him on a campaign against the Thracians. He also received large estates in Greece, including 10,000 iugura (6,600 acres) of land in Boeotia, and was referred to as a friend and ally of the Roman people. His fleet was immobilised or surrendered, and the last Pontic garrisons in Greece withdrawn.
Mithridates accepted most of Sulla's terms, apart from the surrender of his fleet and the evacuation of Paphlagonia. Archelaus was sent to Mithridates to convince him to agree to the remaining terms, but instead was able to set up a meeting between Sulla and Mithridates at Dardanus in Troas. At this meeting Mithridates finally agreed to all of Sulla's terms, agreeing to the Peace of Dardanus.
Archelaus soon fell out of favour at the Pontic court where Mithridates came to believe that Archelaus had made too many concessions to Sulla. In 83 B.C. Mithridates was preparing for a campaign across the Black Sea, but the preparations for this concerned L. Murena, Sulla's successor in Roman Asia. He believed that Mithridates was preparing to attack the Romans, and this belief was confirmed when Archelaus appeared at his camp and made the same claim. Archelaus then convinced Murena to launch a pre-emptive attack on Mithridates, beginning the Second Mithridatic War. After that Archelaus disappears from our sources. We can infer that he remained in favour with the Romans, and that he died before the end of the Third Mithridatic War, for at the end of that war his son, another Archelaus, was rewarded with the temple-state of Comana.