Mithridates VI Eupator (111-63 B.C.), King of Pontus (the area to the north of Armenia, on the shore of the Black Sea), had engaged in two wars against Rome. He was defeated for the third time in 71 B.C. by the Roman army, headed by Lucullus. Mithridates fled to Armenia and asked for asylum at the court of his son-in-law, Tigran.
Tigran had managed to stay neutral in the war. Lucullus wanted to get Mithridates at any price and take him to Rome as a trophy and proof of his major victory. Hence he sent a delegation to the court of Tigran II and demanded the extradition of Mithradates.
Tigran refused to comply with Roman demands, since Mithridates was his guest and it would have gone against the principles of hospitality to surrender him to the enemy. However, at the same time Tigran made it clear that he wished for continued peace and friendship between Rome and Armenia, and assured the delegates that he did not have any plans to expand westward, neither towards Asia Minor nor Pontus.
War between Rome and Armenia was inevitable. Only the Roman Senate had the authority to declare it. But the Senate was doubtful, since the war against Pontus had been going on for eighteen years and Romans were afraid that history would repeat itself in Armenia. Besides that, Rome had regarded Armenia as a friendly major power who indirectly acted as an ally against their former enemy, the Seleucids of Syria, but also as a shield against their potentially greatest enemy, Persia. Therefore, Armenia had proven herself as a shield for the West.
With the Senate unable to decide whether to start a war against Armenia or not, Lucullus took matters into his own hands and begin preparations for war himself. Thus, he violated the laws of the Roman Republic which gave the Senate the exclusive right to make decisions on warfare and foreign policy.
Shortly after receiving Tigran's response, Lucullus began to prepare his army, a task that took him the entire winter of 70-69 B.C.
The Roman general left a legion behind in Pontus in order to maintain the order and began to march towards Melitene (Malatia) with the rest of his army in the spring of 69 B.C. Without declaring war, Lucullus marched over the high land and crossed the Taurus Mountains before the Armenians had had a chance to set up a defensive position in the pass. The Roman army was able to go all the way to Arghana and then move on to finally surround Tigranakert. Tigran was completely surprised by the beginning of the war and by what he probably termed a treacherous attack. After the defeat of a cavalry army of 3,000 he sent to contain Lucullus, Tigran left Tigranakert and hurried to Mush to gather his forces. Meanwhile, the royal palace, highly-walled and well-defended, withstood the Roman siege.
Tigran marched his army from Mush through the Taurus Mountain towards Tigranakert and was able to cover the distance between the two cities, which lay 200 kilometers apart, within ten days. There are different accounts about the size of the two armies, but most claims by Roman historians that Tigran’s army had between 200,000-300,000 soldiers are definitely exaggerated. It may be assumed that Tigran’s army had around 80,000 men, which doubled the size of Lucullus’ army.
Lucullus left 6,000 of his men behind to continue the siege of Tigranakert, while he, together with the rest of his army, began to march towards the Armenian army.
Mithridates had sent one of his best commanders, Taxiles, to Tigran's aid. He suggested that Tigran should refrain from a direct confrontation and let his light cavalry irritate Lucullus' army with sporadic attacks, thereby cutting off contact between the main forces and their supply and ammunition. This way he would starve Lucullus' army – a move that came to be the classical and successful strategy of the Persians. But Tigran, who had noticed the numerical inferiority of the Roman army, answered: "If these are supposed to be an army and are here to fight, then they are too few; if they are messengers to discuss peace, then they are too many."
Since the ground on the other side of the river, where Tigran had his camp, was too steep, Lucullus had chosen to move his army downstream and had taken position in a flat area by the side of the river. The Armenian king thought that the Romans were retreating. However, as soon as the Romans came to a suitable place further down by the river, they crossed it. The Armenian army, mostly composed by non-Armenian forces, had taken position on the opposite side of the river. Tigran was in the core and personally lead the famous heavily armored Armenian cavalry, which was positioned at the rear end of the right flank. This was a fatal mistake, since it exposed such a heavy-moving unit to a grave danger.
The skillful Lucullus realized that if he could take out the famous Armenian cavalry, then he would paralyze the entire Armenian force. Therefore he attacked the Armenian army from the side with his special cavalry and from the rear with two infantry legions. The Armenian infantry was made for attacking and not for defense. In order to regroup and to make more maneuvering space, the army forces started to rotate to the only side which was free from attack, i.e. to the left, where Tigran's other forces were positioned, and this resulted in total chaos in the entire Armenian army. This was the seed for a catastrophic defeat for Tigran.
The battle ended with the fall of Tigranakert shortly afterwards. The city was plundered and looted; Lucullus was able to capture the enormous treasures of the Armenian court, and also 10,000,000 cubic meters of wheat to provide his hungry army with food.
Despite the defeat, Tigran chose to continue the war and put together a new army; the harsh Armenian winter and the continuous attacks of the army finally triggered a mutiny in the Roman troops and forced Lucullus to withdraw from Armenia in 68 B.C. The Roman general was recalled by the Senate.