The battle took place near Zela (modern Zile), which is now a small hilltop town in the Tokat province of northern Turkey.

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 had defeated Caesar's Legate Gnaeus Domitius Calvinus, and his small Roman and allied army at the Battle of Nicopolis.

Pharnaces had an army of approximately 20,000 strong, mostly consisting of tribal and levied infantry, but also containing a core of professional soldiers: phalangites, legionaries, and cavalry.

His five-day campaign against Pharnaces was 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").

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 Bosporan kingdom in recognition of his aid during the Egyptian campaign.