In the wake of the various conflicts that were taking place to the east of Roman territory, such as the Mithridatic War, Tigranes' wars of conquest, Maues' conquest of Parthia, Roman attention was very much drawn to the area, both from concern about danger to the possibility to exploit the instability brought by those wars.
To this end, the Roman Republic entered into an alliance with the Xanthian Kingdom in 678 AUC (76 BC), against possible threats from their mutual neighbor, the Kingdom of Armenia, still ruled by Tigranes the Great. Both the Romans and Xanthians eyed territory ruled by Tigranes; the Romans sought Syria, while the Xanthians sought northern Mesopotamia. Tigranes, however, played the game of diplomacy well and did not provide either state with a cause for war, even though he himself had a valid cause in the subjugation of Characene, Elymais, and Persia by Maues.
Maues would end up dying in 682 AUC (72 BC), of an old wound sustained in battle, which had grown aggravated in recent years. He was succeeded by his young son, Azes, who had developed a reputation as an intellectual and recluse. In the early months of his reign, he quickly showed great aptitude for administration, but no inclination towards military matters. Tigranes found the opportunity to be too great, and had grown restless in the years, yearning to resume the expansion of his empire. Further, the Roman Republic had committed most its forces to fighting various barbarian tribes along the frontiers. Invoking his duties to protect his allies to the south, Trigranes invaded Xanthian Mesopotamia, quickly marching on Seleucia and Ctesiphon.
Azes dispatched an army under the command of Zeonises, a prestigious aristocrat and general in the Xanthian court. However, Zeonises owed much of his position to political intrigues and heredity, and was not half as skilled a general as he was purported to be. His army, though it should have been more than a match for Tigranes', suffered repeated defeats, bloodied severely with each battle.
Compounding matters was Zeonises' style of command. Though not nearly competent enough for the task ahead of him, Zeonises was, at the very least, courageous, and led his troops into battle himself, throwing himself into the thick of each battle. Here, his skill as a single combatant shone. However, that skill was overshadowed by his mediocre tactical and strategic senses, and provided him with enough arrogance to ignore the advice of those who knew better. Zeonises would be killed in action in a battle near the ruins of old Babylon in early 683 AUC (71 BC), in which his army was utterly routed.
Azes then dispatched another general, Spalahores, to lead a new army against Tigranes. Spalahores was much more cautious than Zeonises, and avoided open battle with Tigranes, preferring instead to attempt to wear down the invading force, which now held Mesopotamia in its entirety. The year 684 AUC (70 BC) proved bittersweeet. In that year, the Roman Republic sent an army under Gneaus Pompeius to Syria, to assist their allies against Tigranes. However, at the same time, a rival rose up against Azes, his cousin, Vonones, who doubted Azes' ability to rule, given his lack of military experience.
Azes was thus forced to lead an army himself against Vonones, while resources were directed from Spalahores to that end. Meanwhile, Tigranes led a portion of his army against Pompeius. Pompeius was nothing if not methodical, and was regarded as something of a logistical genius, having established a very thorough and effective supply train for his legions. He had served with distinction under Sertorius in various campaigns, and now had used his political backing to secure a command of his own.
The Syrian coastal cities fell quickly to a series of assaults and sieges, while Pompeius maintained complete naval dominance, ensuring that he could move his forces quickly and effectively. Combined with extensive scouting, he was well aware of Tigranes' advances, and was able to transfer his troops accordingly, scoring an early victory against Tigranes' outside of Antioch in the first year.
However, around the same time, Azes was having difficulty putting down the revolt by Vonones, having lost consecutive battles to the pretender. Azes decided to conclude the war against Tigranes, ceding Mesopotamia to the Armenian king and confirming the independence of Characene, which quickly fell under Armenian influence. Azes then commanded Spalahores to march his army east, to assist Azes. As it happened, Vonones, knowing that he had to defeat his cousin before the relief army could reach him, launched a surprise attack on the king's forces. However, the battle quickly turned against the pretender army, and Vonones was shot by an arrow and died, leading to the surrender of his forces. Azes was left in complete control of the Xanthian Kingdom, and spent the rest of the year touring the country to solidify his control.
Meanwhile, Tigranes was able to divert more troops against Pompeius, and the two forces met again, several miles west of Palmyra. There, the Roman forces were victorious and Tigranes' army suffered significant casualties and was forced to retreat from Syria, to meet up with reinforcements. Pompeius did not pursue Tigranes', however, preferring to secure the cities of Syria. Thus, Tigranes was able to return with a new army in 685 AUC (69 BC), fighting several inconclusive battles against Pompeius, exhausting both armies, as the battle sites moved inexorably closer to the Armenian homeland.
Eventually, Tigranes concluded that he had little chance of holding onto Syria, and come to terms with the Romans, accepting their conquest of the territory. Syria became a Roman province, and Pompeius earned the cognomen, 'Syricus.' Meanwhile, Tigranes effectively exchanged Syria and access to the Mediterranean for Mesopotamia, and access to the Persian Gulf.
Not much to explain on this one. I would like to mention, however, that Pompeius behaves much as he does in our history. Methodical and cautious, he missed many opportunities to follow up with victories. Reading of his exploits occasionally reminds me of General McClellan.
I decided to follow up Maues, a conqueror in the mold of Alexander (though conquering in the opposite direction), with Azes, an intellectual more concerned with administration and diplomacy; just the kind of king the Xanthians need, but also, the kind that hasn't earned respect from the rowdy warlords that had done all the hard work of conquering the empire in the first place.