The battle of Cnidus (394 BC) was a decisive Persian naval victory that ended the brief period of Spartan naval supremacy that followed the end of the Great Peloponnesian War, and in its aftermath the short-lived Spartan domination of the Aegean crumbled.
At the end of the war Athens had been eliminated as a naval power, and Sparta had a fleet paid for with Persian money. The Spartans took over much of Athens's maritime empire and imposed their own governors. The Spartans didn't cope well with their increased power. They soon alienated their Persian allies, supporting Cyrus the Younger's failed revolt against Artaxerxes II. This triggered the Persian-Spartan War (400-387 BC), which saw a series of Spartan armies campaigning in western Asian Minor. At first these were led by fairly minor figures, but after they failed Agesilaus II arrived to take command.
At about the same time the satrap Pharnabazus, possibly with the help of the exiled Athenian admiral Conon, managed to convince Artaxerxes to fund the construction of a new fleet, to counter the Spartan threat. Command of this fleet was given to Conon, but the money soon ran short. Conon had to visit the Persian Court, where he managed to convince the Emperor to provide more funds. Conon was also allowed to pick a Persian to command the fleet, and picked Pharnabazus.
The new fleet is often described as a joint Persian and Greek fleet, but this is a little misleading. The 'Greek' element was still part of the Persian fleet, and came from cities and shipyards in Asia Minor. Many of its sailors would also have been from that area, although Conon was joined by a sizable number of Athenian exiles and volunteers. The Persian side was based around a Phoenician squadron, representing the other great naval power of the period. By the time of the battle the Persian fleet contained 90 triremes.
After some early successes Agesilaus had been sent reinforcements, including a significant number of triremes, and the authority to pick his own commander. He chose Peisander, his brother in law and a capable, ambitious man who didn’t have much naval experience. By the time of the battle Peisander had 85 triremes in his fleet.
Sparta soon found herself engaged in two wars, after a border incident in central Greece triggered the Corinthian War. Early in this conflict the Spartan leader Lysander was killed in battle at Haliartus (395 BC). Agesilaus was recalled to Greece, and left Peisander in command of the Spartan land and naval forces in Asia.
Diodorus gives some details of the campaign that led to the battle. The Spartan fleet was based at Cnidus, at the western tip of the Carian Chersonese.
Conon and Pharnabazus had their fleet at Loryma, at the southern tip of the Rhodian Chersonese, further east along the coast of Asia Minor, so the two fleets were facing each other across the gulf between the two peninsulas.
The campaign began with Peisander taking his fleet east to Physcus of the Chersonese (modern Marmaris), north-east of Loryma. This meant that he would have taken his fleet right past the Persian base, but Loryma is at the northern end of a deep narrow bay (still with a small jetty at its head), so it isn't hard to imagine the Spartan fleet suddenly appearing as it crossed the mouth of the bay, catching Conon and Pharnabazus by surprise.
According to Diodorus the battle happened after Peisander left Physcus and ran into the enemy fleet, but he doesn't say in which direction the Spartans went after leaving port. Xenophon gives a much briefer account of the battle, and just says that it took place off Cnidus. The most likely course of events is that the Spartans had turned back, and were heading west towards Cnidus, and ran into the Persian fleet which was heading east to try and catch them.
Both of our sources agree that the battle began with Conon's squadron of the Persian fleet in front, and Pharnabasus with the Phoenicians in the rear. They also agree on the overall course of the battle, and only disagree in small details.
The fighting began with a clash between Conon's squadron of the Persian fleet and Peisander's fleet. According to Diodorus the Spartans had the advantage in this part of the battle, and if all of the Spartan fleet was engaged then Conon would have been outnumbered. Xenophon states that Peisander could clearly see that his fleet was smaller than Conon's formation, but this seems unlikely. It is possible that he was distinguishing between the Spartan and Peloponnesian core of the fleet and the Allied contingents.
The battle turned when the Phoenician fleet under Pharnabazus entered the fighting. Sparta's allies, on the left of the fleet, fled to land, leaving the Spartans to fight on alone. The Spartans fought on, but were eventually forced to land themselves and most of the crews escaped. Peisander refused to run, and was eventually killed fighting on his own ship. According to Diodorus the allies captured fifty Spartan triremes and 500 crewmen, but the rest of the crews escaped. The remaining 35 triremes also escaped and made it back to Cnidus. Xenophon also has most of the crewmen escaping, and reaching safety at Cnidus overland.
We can only speculate about the exact division of ships. One possibility is that Peisander had 50 Spartan and 35 allied ships. If the Spartans were outnumbered by Conon's force then the allied fleet might have been split between 60 Greek manned ships and 30 Phoenician ships - neither source gives a breakdown of this fleet. Peisander's 85 had the edge over Conon's 60, but when the Phoenicians arrived and the Allies fled that would have left him with 50 ships to face 90, the sort of odds that would explain the severity of the defeat.
The aftermath of the battle was more important than the fighting itself. The Spartans had clearly made themselves rather unpopular amongst the Greeks of Asia Minor, and as Conon and Pharnabazus cruised along the coast, most cities expelled their Spartan governors. Their cause was helped by Pharnabazus's promise not to leave Persian garrisons in any of the cities. Spartan power collapsed around most of the coast of Asia Minor and on the Aegean Islands. Only around the Hellespont did they retain some territory.
In 393 Conon and Pharabazus crossed the Aegean to Greece, where they raided the Peloponnese and visited Corinth to encourage the anti-Spartan coalition in the Corinthian War. Conon's proudest moment must have been his return to Athens, where he provided money and men to help rebuilt the walls of Piraeus and the Long Walls, torn down less than a decade earlier at the end of the Peloponnesian War.
News of the defeat at Cnidus reached Agesilaus just before the battle of Coronea. He realised that a large part of his army was made up of Greek allies who would probably soon desert his cause, and so he told them that the Spartans had won the battle, although he did admit that Peisander had died in the fighting. He then went on to win an inconclusive victory at Coronea, before disbanding his army.
The battle of Cnidus changed the balance of power across the Aegean. Although Sparta emerged as the victors in the Corinthian War, their power was now limited to mainland Greece. They did manage to get smaller fleets into Asia waters after Cnidus, but never threatened to restore their dominance of that area.