The battle of Zela took place on 2 August 47 (on the Roman calendar; 21 May 47 BCE on ours). An account was written by an anonymous officer, maybe Caesar's lieutenant Aulus Hirtius. Sections 65-77 of the Alexandrine War were translated by W. A. McDevitte and W. S. Bohn.
When Caesar arrived in Syria, from Egypt,note[He had spent the winter there with Cleopatra.] and learned that the government at Rome was badly and injudiciously conducted [...], thought it was yet first incumbent upon him to settle the state of the provinces through which he passed [...]. This he hoped soon to effect in Syria, Cilicia, and Asia, because these provinces were not involved in war. In Bithynia and Pontus indeed he expected more trouble, because he understood Pharnacesnote[Son of Rome's archenemy Mithridates VI of Pontus.] still continued in the latter, and was not likely to quit it easily, being flushed with victory [...].
After a stay of some days in these parts, he named Sextus Julius Caesar, his friend and relation, to the command of Syria and the legions appointed to guard it; and sailed himself for Cilicia, with the fleet he had brought from Egypt. [...] Advancing through Cappadocia [...], he stopped two days at Mazaca and arrived at Comana. [...] Upon his approaching Pontus, and the frontiers of Galatia, Deiotarus, ruler of that province [...], came in a suppliant manner to Caesar, to beg forgiveness for assisting Pompey with his army [during the battle of Pharsalus] [...]. Caesar restored him and commanded him to join him with all his cavalry, and the legion he had trained up after the Roman manner.
Caesar arrived in Pontus and drew all his forces together, which were not very considerable, either for their number or discipline. The Sixth Legion, which was composed of veterans that he had brought with him from Alexandria, [had been] reduced to less than a thousand men. He also had the legion of Deiotarus, and two more that had been in battle [...].
Ambassadors arrived from Pharnaces, "to entreat that Caesar would not come as an enemy, for he would submit to all his commands." [....] Pharnaces promised every thing, but [tried] to elude his engagements. Caesar, perceiving his drift, did now, out of necessity, what he was usually wont to do through inclination, and resolved to decide the affair as soon as possible by a battle.
Zela is a fortified town in Pontus [...]. All around is a great number of large mountains, intersected by valleys. The highest of these, which is celebrated for the victory of Mithridates, the defeat of Triarius, and the destruction of our army, is not above three miles from Zela, and has a ridge that almost extends to the town. Here Pharnaces encamped, with all his forces, repairing the fortifications of a position which had proved so fortunate to his father.note[The Pontian king had chosen Zela to block Caesar's advance into his kingdom. Zela controled access to the most important pass across a mountain range that covered Amasia, tha capital, as a wall.]
Caesar having encamped about five miles from the enemy, and observing that the valleys which defended the king's camp would likewise defend his own, at the same distance, if the enemy, who were much nearer, did not seize them before him, ordered a great quantity of fascines to be brought within the entrenchments. This was quickly performed and next night, at the fourth watch, leaving the baggage in the camp, he set out with the legions; and arriving at daybreak unsuspected by the enemy, possessed himself of the same post where Mithridates had defeated Triarius. Hither he commended all the fascines to be brought, employing the servants of the army for that purpose, that the soldiers might not be called off from the works [...].
Pharnaces perceived this, and next morning ranged all his troops in order of battle before his camp. Caesar, on account of the disadvantage of the ground, believed that the king was reviewing them according to military discipline, or with a view to retard his works, by keeping a great number of his men under arms, or through the confidence of the king, that he might not seem to defend his position by his fortifications rather than by force. Therefore, keeping only his first line in order of battle, Caesar commanded the rest of the army to go on with their works.
But Pharnaces, either prompted by the place itself, which had been so fortunate to his father; or induced by favorable omens, as we were afterward told, or discovering the small number of our men that were in arms (for he took all that were employed in carrying materials to the works to be soldiers), or confiding in his veteran army, who valued themselves upon having defeated the Twenty-Second Legion;note[Of Deiotarus.] and at the same time, despising our troops, whom he knew he had worsted [...], was determined upon a battle, and to that end began to cross the valley. Caesar, at first, laughed at his ostentation, in crowding his army into so narrow a place, where no enemy, in his right senses, would have ventured: while, in the mean time, Pharnaces continued his march, and began to ascend the steep hill on which Caesar was posted.
Caesar, astonished at his incredible rashness and confidence, and finding himself suddenly and unexpectedly attacked, called off his soldiers from the works, ordered them to arms, opposed the legions to the enemy, and ranged his troops in order of battle. The suddenness of the thing occasioned some terror at first. Our ranks were not yet formed, when the scythed chariots disordered and confused our soldiers. However, the multitude of darts discharged against them soon put a stop to their advance.
The enemy's army followed them close, and began the battle with a shout. Our advantageous situation, but especially the assistance of the gods, who preside over all the events of war, and more particularly those where human conduct can be of no service, favored us greatly on this occasion.
After a sharp and obstinate conflict, victory began to declare for us on the right wing, where the sixth legion was posted. The enemy there were totally overthrown, but, in the center and left, the battle was long and doubtful; however, with the assistance of the gods, we at last prevailed there also, and drove them with the utmost precipitation down the hill which they had so easily ascended before. Great numbers were slain, and many were crushed by the flight of their own troops [...]. Our victorious men did not hesitate to advance up the disadvantageous ground and attack their fortifications, which they soon forced, notwithstanding the resistance made by the troops left by Pharnaces to guard it. Almost the whole army was cut to pieces or made prisoners. Pharnaces himself escaped, with a few horse; and had not the attack on the camp given him an opportunity of fleeing without pursuit, he must certainly have fallen alive into Caesar's hands.
Though Caesar was accustomed to victory, yet he felt incredible joy at the present success, because he had so speedily put an end to a very great war. The remembrance, too, of the danger to which he had been exposed, enhanced the pleasure, as he had obtained an easy victory in a very difficult conjuncture. Having thus recovered Pontus and abandoned the plunder of the enemy's camp to the soldiers, he set out next day with some light horse. He ordered the sixth legion to return to Italy to receive the honors and rewards they had merited; and sent home the auxiliary troops of Deiotarus, and left two legions with Caelius Vincianus to protect the kingdom of Pontus.