Mithridates VI of Pontus came to power in the Hellenized region of Asia Minor circa 121 and 120 BC. Murdering his brother, mother and other potential rivals he established himself as sole ruler, with intentions of expansion at the expense of Rome.
By the 90's BC, Mithridates took firm control of several neighboring regions to the north and continued efforts would push him into direct conflict with Rome. The first matter of dispute was with the Roman ally of Bithynia over territory in Phrygia. Next he attempted to take control of Cappadocia where Sulla had recently placed the Roman client king, Ariobarzanes on the throne.
A lack of Roman interference on these minor issues probably played a part in pushing Mithridates agenda farther. By 91 BC, Mithridates supported the overthrow of the Bithynian King Nicomedes and then influenced local forces to overthrow Ariobarzanes in Cappadocia. Just at the beginning of the Social War in Italy, the Roman Senate responded with a delegation led by M. Aquillius.
Charged with diplomatically resolving the various regional disputes, Aquillius managed to convince Mithridates for the time being, to restore his neighbors to their previous situations. Conveniently finding that he wasn't directly involved, Rome left Mithridates with the tag of 'friend of the Roman people.' Whatever additional discourse was taking place is unclear however. In the heat of the social war Rome was unable to interfere directly with military force, and it seems that Aquillius convinced Nicomedes to wage war on Pontus. While it was likely a combination of personal greed and some concept of assessing Mithridates strength, both Aquillius and Nicomedes were soon surprised to learn that this strength was formidable indeed.
Mithridates' original attempt to negotiate with diplomacy ended in rebuttal by Aquillius and he had little choice but to respond. In 90 BC, Mithridates invaded Cappadocia and the minor regional dispute was quickly turning into a full scale war. With little military strength to work with, three small and separate forces were utterly destroyed by Mithridates. All resistance to the Pontic army collapsed and it swept into Asia Minor and neighboring territories.
In a very short time, Mithridates expelled what small Roman forces were available, taking control of Asia Minor, Bithynia, Cappadocia, the Bosporus and the Black Sea. Much like Hannibal's attempts earlier in Italy, Mithridates encouraged the locals to join with him but with far greater success. Aquillius himself was captured in Mitylene while fleeing to Rhodes. As punishment for Roman excess (and a recurring event in ancient written history which is probably simply intended to symbolize and emphasise greed), he was supposedly executed by pouring molten gold down his throat.
At this point, despite not actually facing any real Roman legions, Mithridates felt confident in his ability to beat Rome. 88 BC was a major offensive which saw the invasions of Rhodes and Greece. While attacks on Rhodes were unsuccessful, Mithridates' general, Archelaus had more success in his attack on the Greek mainland.
First securing Athens, much of southern Greece was brought under Pontic control within a short time, with the local populations initally happy to be outside of Roman influence. In Central Greece, Mithridates met some resistance from Q. Braettius Sura whose legion guarded Macedonia from the Illyrian tribes stopping an invasion of the central regions leading to an encounter with Archelaus in a battle at Chaeronea. A moderate victory sent Archelaus back to Athens for the winter, but reinforcements strengthened his position and prevented follow-up by Sura.
Meanwhile, Mithridates laid out a plan to ensure the loyalty of his newly won regional allies. He issued a proclamation ordering the massacre of all resident Romans in Asia Minor and surroundings. All in all, up to 80,000 Roman citizens were slaughtered. While certainly drawing Rome's wrath, Mithridates forced the loyalty of these new conquests by ensuring they wouldn't be eager to face Roman vengeance.
By 87 BC, Sulla, having been appointed to Consular command of the expedition, only to have it taken away by Marius, and then reaffirmed by marching on Rome, was ready to face Mithridates. While Sulla was gathering strength on his move, the Macedonian governor Sentius recalled Sura to grant Sulla full control of the campaign. Conveniently, Mithridates launched an attack on Macedonia that was repulsed due to Sura's timely recall. With 5 legions and whatever local forces he could muster, Sulla was now in complete command and able to fully concentrate on re-establishing control of the region from the Mithridatic forces.