The Aquillian Legation, 90-89 BC
In the late summer 90 BC a Senatorial legation was sent east, under Mn. Aquillius and Manlius Maltinus, to restore Nicomedes and Ariobarzanes to their kingdoms. The Senate also sent instructions to Cassius "the commander of Asia about Pergamon who had a small army" and to Mithradates Eupator himself to assist in this.
Cassius' small army was probably the standard peace-time garrison force of between a whole and half legion (5 to 10 cohorts) and a few local auxiliary units. Certainly no more than 5,000 troops in all. The Aquillian legation soon augmented it with a large force of Galatian and Phrygian auxiliary regiments and with these troops proceeded to restore both monarchs. Mithradates, angry with the Romans, refused to cooperate but neither did he offer opposition and both kings were restored without any fighting in about the autumn 90 BC.
Its mandate achieved, the Aquillian legation ought to have gone home in winter 90-89. Instead, no doubt on the excuse of keeping Mithradates under observation, it began to work upon Marius' covert instructions to Aquillius of provoking the Pontic King to war. A very risky and reckless policy with the Italic War still in the balance.
The kings, Nicomedes in particular, had taken out big loans at Rome to bribe the Senators to vote for their restoration (this decision was a given in accordance with long-term policy in the region, but it appears that by now nothing much was done by the Senate in foreign affairs without accompanying payments from the foreigners with something to gain by Roman intervention). Aquillius' retinue included representatives of the lenders. With Aquillius' support they now urged the two kings to invade the Pontic kingdom to secure the booty with which to repay the bribery loans. Fearing the power of Mithradates (and probably aware that the Senate had given no such orders), both kings demured. But Nicomedes' creditors persisted with their pressure until he at last consented.
It was probably at the end of autumn 90 that Nicomedes regained control of the Thracian Bosporos and in the new sailing season (from mid March 89 BC) he prevented egress from the Euxine to Pontic ships.
Around the middle of spring 89 Nicomedes invaded the ancient Mithradateian dynastic lands of Mariandynia, plundering as far east as Amastris without encountering resistance. Mithradates had long been preparing a challenge to Roman power and the time was now ripe. As a final means of enlisting as much sympathy as possible in Anatolia, he offered no opposition to the Bithynian raid, preferring to appear as manifestly wronged by the puppets and representatives of Rome. The Bithyni returned home with a great deal of plunder - presumably sufficient for Nicomedes to repay his debts.
After the raid Mithradates sent his spokesman Pelopidas to the Roman legates and commanders to make complaint, apparently at Pergamon. At the same time Mithradates went on with his war preparations, trusting especially in his existing alliance with Tigran of Armenia, although the more distant connection with Parthia was now without use because his ally Mithradates II had been slain by his rival Sanatruk attacking from the east in summer 91 BC, and a serious internal war persisted between Sanatruk and Mithradates' eldest son and heir Gotarzes I. Eventually the Parthian internal conflict was to seize the entire attention of Tigran too, but this could not yet be known. The Pontic king was also exploiting carefully prepared networks of support and recruitment among the Thracians and the Scythians, and now solicited help and alliance from the kings in Syria and from Ptolemy Alexander I and the Cretans.
The Pontic envoy Pelopidas cleverly ignored the fact that Aquillius and his suite had induced the Bithynian raid. Instead he made propaganda about Roman intolerance towards Mithradates and concluded by appealing to the Treaty between Mithradates and Rome, calling upon the Romans, as friends and allies, to punish or restrain the Bithynian aggressor. Bithynian envoys replied first, citing Pontic aggression against Bithynia and her present king, the ominous Pontic build up of arms, territory and resources, and alliances - from Armenia to Thrace while negotiations were still in progress with the Ptolemaic Empire and Seleucid Empire. Such vast preparations, the Bithyni insisted, were aimed not at Bithynia but at Rome herself. Pelopidas countered by agreeing to let bygones be bygones, and accepting all Roman acta in the East hitherto. But he insisted that something must be done about the most recent Bithynian acts of aggression: the closing of the Euxine and the invasion and plunder of Pontic territory. He once again called upon the Romans to honour the letter of the Treaty and help Mithradates punish his attackers, or at least its spirit and to stand aside while Mithradates himself took his revenge.
Through Pelopidas' skill in presenting the case, Mithradates' attempt to embarrass and even discredit the Roman representatives succeeded. The latter had made a show of listening fairly to both sides and were now embarrassed by the obvious injustice done to a nominal friend and ally. After a lengthy delay they finally came up with a publicly acceptable pronouncement: we do not wish harm done to our ally Mithradates, nor can we allow war to be made against Nikomedes because it is against the interests of Rome that he be weakened. Assembly dismissed. Pelopidas wished to make something of the insufficiency of this answer, but was ushered out.
Read more about this topic: First Mithridatic War