The Social War (357-355 BC) was a conflict between Athens and a number of key members of the Athenian League. The war significantly weakened Athens, and also meant that she was unable to intervene as Philip II of Macedon expanded his kingdom.
The period of Theban hegemony ended after the battle of Mantineia (362). Although the Thebans won this battle, their inspirational leader Epaminondas was killed. He had no successor, and an uneasy peace descended on parts of Greece. The wars of the last few years had greatly reduced the power of Sparta, which had lost the Peloponnesian League and much of its mainland empire and was now more concerned with the dangers of a helot revolt. Athens appeared to be more powerful, at the head of a revived Athenian League with over seventy members, but she also faced a series of threats.
The main threats to Athens were in the north. They were engaged in a small scale war with Cotys, king of Thrace, at least until his murder in 359. Alexander of Pherae in Thessaly was another threat, with a reasonably powerful fleet. Behind them all was Macedonia, which protected the new Chalcidice League, and with it the city of Amphipolis, which Athens had founded but almost immediately lost, and now coveted. In the west Corcyra on Corfu left the league in 361.
A more dangerous threat came in 357 when the Thebans stirred up a revolt on the island of Euboea, north of Attica. Control of this island was essential for Athens, as it sat across their supply lines from the Black Sea. The revolt on Euboea was quickly put down, but the Athenians then attempted to tighten their control over the Athenian League. This triggered a wider crisis, the Social War.
One of the reasons for the collapse of the first Athenian Empire was the increasing arrogance of the Athenians, which undermined the basis of the league. The rules of the new Athenian League had been designed to try and prevent the same happening again, but the league members must have been watching for any backsliding. In 365 Athens had taken Samos into the league, and placed colonists there. Soon afterwards Ceos and Naxos lost their judicial independence and became subject to the Athenian courts.
In the autumn of 357 Chios refused to pay its contribution to the league treasury. Chios then formed an alliance with Rhodes and Byzantium, and gained the support of Mausolus, the semi-independent satrap of Caria. Cos also quickly joined the revolt, which thus involved powers spread out along the entire western coast of Asia Minor, as well as the vitally important route into the Black Sea.
Mausolus was an ambitious ruler, who had his eyes on the Greek islands near his coastline. Demosthenes considered him to have been the prime mover behind the revolt (On the Liberty of the Rhodians).
The first actions of the war took place at Chios, about half way up the western coast of Asia Minor, and the closest of the rebel powers to Athens. The rebels concentrated their forces at Chios. The Athenian forces, led by Chares and Chabrias, also made for Chios. The resulting battle of Chios (357 or 356 BC) was a joint land and sea attack on the town of Chios. Both attacks failed, and Chabrias, who commanded the naval part of the attack, was killed in the battle.
In the aftermath of this defeat Sestos and other towns joined the revolt. A rebel fleet of 100 ships attacked Lemnos and Imbros, two key Athenian possessions in the northern Aegean. They then moved south to besiege Samos. Chares, who only had sixty ships, was unable to intervene.
In 356 BC Iphicrates and Timotheus were given a fleet, partly funded by a new war tax, and were sent to deal with the rebels. They took their fleet towards Byzantium, in an attempt to lift the siege of Samos. They succeeded in that aim, but then suffered a defeat at sea (battle of Embata). In the aftermath of this battle Chares prosecuted Iphicrates and Timotheus, blaming them for the defeat. Iphicrates was acquitted (at least according to Cornelius Nepos), but never given another command. Timotheus was fined 100 talents, and went into exile.
This just left Chares, but he was short of supplies and money. In an attempt to gain both he offered his support to Artabazus, satrap of Hellespontine Phrygia, who was then involved in a revolt against Artaxerxes III (Satrap's Revolt). Chares won a victory for Artabazus, and was rewarded with more than enough money to pay his troops, as well as Sigeum and possibly Lampsacus.
The war was ended by Artaxerxes III, the last really influential Persian monarch. He sent an ultimatum to Athens, demanding that they recalled Chares. Athens was now exhausted, and had little choice other than to comply, especially once news arrived that Artaxerxes was preparing a large fleet.
The war was ended by a second King's Peace. Athens had to recognise the independence of Byzantium, Chios, Rhodes, Cos and Corcyra. Mytilene, Perinthus, Selymbria and Methymna took advantage of the peace to leave the League. Athens retained some islands in the Cyclades, a few northern harbours, and freedom of navigation in the Aegean.
Cos and Rhodes probably soon regretted their actions. With Athenian protection gone, their democratic governments were vulnerable, and they were soon replaced with oligarchies imposed by Mausolus. Another oligarchy came into power at Chios. Soon after the death of Mausolus in 353 the exiled Rhodian democrats asked Athens for help, but unsurprisingly the request was turned down.
The Social War was very quickly followed by the Third Sacred War (355-346 BC), which saw the start of the rise of Macedonia to a dominant position in the Greek world. Philip II had already taken advantage of Athens's focus on the Social War to take Amphipolis (357 BC), and then to capture Potidaea (356 BC) in order to secure a short term alliance with Olynthus.