The First Mithridatic War (89–85 BC) was a war challenging the Roman Republic's expanding empire and rule over the Greek world. In this conflict, the Kingdom of Pontus and many Greek cities rebelling against Roman rule were led by Mithridates VI of Pontus against Rome and the allied Kingdom of Bithynia. The war lasted five years and ended in a Roman victory, which forced Mithridates to abandon all of his conquests and return to Pontus. The conflict with Mithridates VI later resumed in two further Mithridatic Wars.
Prelude
Following his ascension to the throne of Kingdom of Pontus, Mithridates VI of Pontus focused on expanding his kingdom. Mithridates' neighbors, however, were Roman client states, and expansion at their expense would inevitably lead him to conflict with Rome. After successfully incorporating most of the coast around the Black Sea into his kingdom, he turned his attention towards Asia Minor (in particular, the Kingdom of Cappadocia) where his sister Laodice was Queen. He had his brother-in-law, Ariarathes VI, assassinated by a Cappadocian nobleman, Gordius, who had been an ally of Mithradates. The kingdom was thus left in the hands of Laodice, who continued the rule as regent for her son Ariarathes VII of Cappadocia.
Laodice married Nicomedes III of Bithynia, whose country was Pontus' traditional enemy. Nicomedes occupied Cappadocia and Mithridates retaliated by driving him out of Cappadocia and establishing himself as patron of his nephew's kingship on the throne. When Ariarathes refused to welcome Gordius back, Mithridates invaded Cappadocia again and killed Ariarathes. He proceeded to place his son, also called Ariarathes, on the throne of Cappadocia under the guardianship of Gordius.
Nicomedes appealed to the Roman Senate, which decreed that Mithridates be removed from Cappadocia and Nicomedes be removed from Paphlagonia and the Senate appointed Ariobarzanes I of Cappadocia as King of Cappadocia. Mithridates prompted his son-in-law Tigranes the Great of Armenia to invade Cappadocia and remove Ariobarzanes.
The Senate sent special orders to Lucius Cornelius Sulla, the propraetor who was in charge of reducing the pirates infesting Cilicia (south of Cappadocia), and charged him with driving out Mithridates's adherents and the Armenians. After initial difficulties Sulla succeeded and Ariobarzanes was restored to his throne.
In Bithynia Nicomedes III had died. He was succeeded by his son Nicomedes IV. Unfortunately for Nicomedes IV, his bastard half-brother, Socrates Chrestus, supported by Mithridates drove him from his kingdom. Nicomedes fled to Rome and got the support of the Romans who promised to restore him to his throne.
Mithridates main ally, his son-in-law Tigranes, had once again invaded Cappadocia and driven Ariobarzanes from his throne.
The Aquillian legation, 90–89 BC
In the late summer 90 BC a Senatorial legation was sent east, under Manius Aquillius and Manlius Maltinus, to restore Nicomedes and Ariobarzanes to their kingdoms. The Senate also sent instructions to Cassius, the Roman governor (probably a propraetor) of the Roman province of Asia, who had a small army and to Mithridates Eupator himself to assist the restorations.
Cassius' small army was probably the standard peacetime 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. Mithridates, angry with the Romans, refused to cooperate but neither did he offer opposition and both kings were restored without any fighting in autumn 90 BC.
Its mandate achieved, the Aquillian legation ought to have gone home in winter 90/89 BC. Instead, no doubt on the excuse of keeping Mithridates under observation, it began provoking the Pontic King to war. This was considered to be a very risky and even reckless policy with the Italian War still in the balance.