The third Roman war against Mithridates VI, King of Pontus, was really a continuation of the second. Lucullus, the Roman legate in charge of carrying out the war effort, was semi-successful, but ultimately unable to win a final victory. By 67 BC, he had been replaced by the Consul Glabrio, through the efforts of the tribune Gabinius.
Glabrio however, would never take part in this command other than to prepare the legions for his replacement, Gnaeus Pompeius Magnus. After Pompey's successful campaign against the Cilician pirates, the Tribune C. Manilius proposed and passed an all encompassing law granting Pompey ultimate authority, and command, in the east.
In 66 BC, Pompey moved to Asia Minor and took control. Though Glabrio and Lucullus were still there, it was Pompey who had ultimate authority. With 8 mostly veteran legions, Pompey marched north east from Asia into Bithynia and farther east into Pontus. Mithridates had vast wealth from his own campaigns at his disposal, but many years of continuous war had taken its toll. Recruitment of local troops was limited, and mercenaries simply weren't available in the numbers he needed. By the time Pompey arrived in Pontus, Mithridates was outnumbered against the Romans for likely the first time. He made a desperate attempt to stop Pompey's advance in what would turn out to be his last battle of importance.
Near the town of Dastira, the two armies met in a fateful battle where Pompey's legions crushed Mithridates. He was forced to abandon his kingdom and Pompey would later establish the city of Nicopolis (Victory City) on the site in commemoration. With a small remaining force, Mithridates, the greatest enemy of Rome since Hannibal, fled east to Armenia. King Tigranes, his son-in law and ally, realized that Mithridates had fought his last fight against Rome, and turned him away. He next fled north to Colchis, then Crimea, where he would spend the next few years trying to assert his influence. Unable to ever really regain his former strength, and losing authority to one of many sons, he eventually took his own life in 62 BC.
Instead of pursuing Mithridates directly, Pompey next took on the challenge of bringing the entire eastern world into the Roman fold. He moved on to Armenia where he met with Tigranes. The King of Armenia immediately bowed to Roman power and surrendered his crown to Pompey, but he was restored to the thrown and established as a client state. Tigranes and the Armenians would prove a vital ally in checking power of the neighboring Parthians. Pushing farther east and north, into the Caucasus mountains and beyond to the Caspian Sea, Pompey established friendly relations with the natives there. Though the intention was not conquest, this campaign was another vital step in securing allies against the Parthians. The natives of the Caucasus region would never display any serious enmity towards Rome and often were supportive during various eastern campaigns.