Phormio son of Asopius (c.470-429/428 BC) was a successful Athenian admiral in the early stages of the Great Peloponnesian War, best known for his two naval victories won in the Gulf of Corinth in 429 BC.
Phormio first appears in Thucydides in 440 BC, during the siege of Samos, when he was one of three generals (alongside Thucydides himself and Hagnon) who brought reinforcements to the Athenian forces involved in the siege.
In 432 BC the Athenians began a siege of Potidaea. The city was built at the northern end of an isthmus, and at first the Athenians only blockaded the city from the north. Phormio was sent from Athens with 1,600 fresh troops and orders to establish a blockade from the south. This force landed some way to the south of Potidaea and advanced up the isthmus of Pallene to the city, apparently hoping that the defenders would come out of the city and fight an open battle. When this didn’t happen he built a line of fortifications to the south of the city, completing the blockade.
Phormio and his 1,600 men didn't stay at Potidaea to take part in the siege, but instead passed around the city and operated in Chalcidice and Bottiaea, on the larger peninsula to the north of the city. In the following year (431 BC) he operated alongside Perdiccas, king of Macedonia, in another campaign against the Chalcidians. In the summer of 430 BC Hagnon was sent to Potidaea at the head of another Athenian army, bringing the plague with him by Athens. By this date Phormio and his 1,600 men were no longer in the area, although their location is unclear.
Phormio next appears in Thucydides at Amphilochian Argos, a city at the eastern end of the Ambracian Gulf, in the north-west of Greece. He is mentioned in the middle of an account of fighting around the city in the late summer of 430, but in a context that suggests that the events being described actually happened much earlier, before the outbreak of the Great Peloponnesian War. At some point in its past the people Amphilochian Argos had invited the Ambraciots, the inhabitants of a Corinthian colony located to the north of the Ambracian Gulf, to join them. Eventually the Ambraciots had driven the original inhabitants out of the city. The dispossessed Amphilochians and their Acarnanian allies called for help from Athens, which sent a fleet of thirty ships under the command of Phormio. The combined Athenian, Amphilochian and Acarnanian army took Amphilochian Argos by storm. The Ambraciots captured in the city were enslaved, and the city was repopulated by the Acarnanians and Amphilochians. The alliance between Athens and Acarnanian, which apparently already existed at the start of the Great Peloponnesian War, dated from this expedition.
Some later authors mistakenly place Phormio's expedition to Amphilochian Argos in the late summer of 430, when the Ambraciots made a renewed attempt to occupy Amphilochian Argos. The confusion has arisen because Thucydides placed his account of Phormio's earlier expedition in the middle of his account of the events of 430, in a passage that explains why the Ambraciots decided to attack Amphilochian Argos in 430. This expedition occupied the countryside around the city, but failed to capture Amphilochian Argos and the Ambraciot army returned home and dispersed.
Phormio returned to the same general area in the winter of 430-429 BC in command of a fleet of 20 triremes. He was based at Naupactus, at the western end of the Gulf of Corinth, and had orders to prevent any hostile ships from travelling to or from Corinth, at the eastern end of the gulf. In reality his fleet was rather too small for this task, and only a certain amount of luck (combined with the skill of the Athenian sailors) prevented it from being swept aside during 429 BC. In that year the Peloponnesians decided to launch an invasion of Acarnanian, starting from Leucas, an allied city at the north-western end of Acarnanian. Part of the expedition came from the Peloponnese, and successful reached Leucas, although the expedition ended in defeat at Stratus.
The other part of the expedition was to come from Corinth, and so had to try and get past the Athenian blockade. Phormio's squadron of 20 warships was outnumbered more than two-to-one by the 47 ships in the Peloponnesian fleet, but he still won an significant victory in the battle of Chalcis (429 BC), in which the Peloponnesians tried to form a defensive circle of ships, which a combination of daring Athenian tactics and a strong wing disrupted. Despite this initial victory Phormio was soon faced by a much larger Peloponnesian fleet, this time of 77 warships. This time it was Phormio who was forced to fight, and the first part of the resulting battle of Naupactus went against him, with nine of his twenty ships forced onto the shore in the first phase of the battle. The surviving eleven ships were then pursued by the fastest twenty Peloponnesian ships. These were better odds, and in a fight outside Naupactus the Athenians were victorious. A number of Peloponnesian ships were captured, and most of the Athenian ships lost in the first phase of the battle were recovered. Soon after this second victory Phormio received reinforcements, and the Peloponnesians dispersed their fleet, surrendered control of the Gulf of Corinth to the Athenians.
Phormio's next campaign was his last one. In the winter after the battle of Naupactus he led a small expedition into Acarnanian, with 400 Athenian hoplites and 400 Messenians. This expedition restored one of Athens's allies (Cynes son of Theolytus, ruler of Coronta) and expelled a number of people judged to be unreliable from various Acarnanian towns. The town of Oeniadae, the only one in Acarnania that had been consistently anti-Athenian, was judged to be too strong to be attacked by the small expedition. After this Phormio returned to Naupactus, before in the spring of 428 BC he and his Athenians sailed back to Athens.
Phormio was probably dead by the summer of 428 when the Acarnanians asked for his son or a close relative as the new commander in their area, although Thucydides doesn't actually mention his death. Phormio's son Asopius was sent out in his place, but was much less successful, and was killed after an unsuccessful attack on Leucas. Phormio's tomb was built on the road to the Academy in Athens, near to those of Pericles and Chabrias.
Phormio had a reputation for being a strict disciplinarian and a man of high standards in public life. He is described by Aristophanes as sleeping on a normal soldier's pallet. At one point, when he was called on to command an expedition to support the Acarnanians, he was in exile having been unable to pay a fine. When he was appointed to command the expedition Phormio pointed out that he wasn't eligible because of the debt. It was illegal to simply cancel the debt, and so instead he was given a simple task to perform, and paid the amount needed to clear his debts.