After the defeat of the Ptolemaic forces at the Battle of the Nile, Caesar left Egypt and travelled through Syria, Cilicia and Cappadocia to fight Pharnaces, son of Mithridates VI.

Pharnaces' army marched down into the valley separating the two armies. Caesar was baffled by this move as it meant his opponents had to fight an uphill battle. Pharnaces' men climbed up from the valley and engaged Caesar's thin line of legionaries. Caesar recalled the rest of his men from constructing their camp and hastily drew them up for battle. Meanwhile, Pharnaces' scythed chariots broke through the thin defensive line, but were met by a hail of missiles (pila, the Roman throwing spear) from Caesar's battle line and were forced to retreat. Caesar launched a counter-attack and drove the Pontic army back down the hill, where it was completely routed. Caesar then stormed and took Pharnaces' camp, completing his victory.

It was a decisive point in Caesar's military career - his five-hours campaign against Pharnaces was evidently so swift and complete that, according to Plutarch (writing about 150 years after the battle) he commemorated it with the now famous Latin words reportedly written to Amantius in Rome Veni, vidi, vici ("I came, I saw, I conquered"). Suetonius says that the same three words were displayed prominently in the triumph for the victory at Zela. Pharnaces escaped from Zela, first fleeing to Sinope then back to his Bosporan Kingdom. He started to recruit another army, but was soon after defeated and killed by his son-in-law Asander, one of his former governors who had revolted after the Battle of Nicopolis. Caesar made Mithridates of Pergamum the new king of the Bosporian kingdom in recognition of his aid during the Egyptian campaign.