The siege of Phlius (381-380/379 BC) saw the Spartans besiege one of their allies in order to restore the rights of a group of exiled oligarchs, one of a series of heavy handed Spartans interventions in the internal affairs of other Greek cities that came in the aftermath of the end of the Corinthian War.
Like many Greek cities, Phlius suffered from an ongoing struggle between democrats and oligarchs. During the Corinthian War the democrats were in charge, and many of the former oligarchs were in exile, and their property confiscated. In 394 BC Phlius refused to contribute troops to the Spartan army, using a religious festival as an excuse, and thus missed the battle of Nemea. Two years later the city was threatened by the Athenian forces of Iphicrates, and had asked the Spartans to occupy the citadel to protect the city from attack. Once the danger was over the Spartans left. After the end of the war the Exiles asked Sparta to try and get them readmitted to the city. The Phliasians reluctantly agreed to allow the exiles home, and restored their property, but not all of their legal rights.
In 381 King Agesopolis was sent north to try and bring the Spartan war against Olynthus to a successful conclusion. Phlius donated a significant amount of money to his war chest, and gained his gratitude. This may have encouraged the democratic hard liners, who now refused a series of demands from the returned exiles. The exiles wanted their rights to be restored to them, and demanded that their legal cases should be judged by an impartial court. The citizens of Phlius insisted that any case had to be heard in the cities own courts.
The disgruntled exiles took their case to Sparta. This action angered the democrats, who imposed a fine on them for this action. The exiles feared to return home, and instead asked for military aid from Sparta. The Spartan ephors supported their case, and ordered an attack on Phlius,
Command of the expedition was give to King Agesilaus II, who had family connections to some of the exiles. As the Spartan army advanced towards the city, a series of embassies arrived from the democrats, attempting to avert the attack. Eventually Agesilaus offered to abandon the attack if they would allow the Spartans to occupy their acropolis, a move that would have given Spartan effective command of the city. These terms were rejected.
At the start of the siege Agesilaus built a line of fortifications around the city, and introduced a blockade. At first the war wasn't popular in Sparta, but as time went on an increasing number of Phliasians came out to join the exiles. Agesilaus made sure that they were supported, as long as they underwent Spartan military training, and eventually there were enough exiles to make a unit of 1,000 men. The high quality of this unit helped win over public opinion in Sparta.
The siege of Phlius dragged on much longer than the Spartans had expected. The Phliasians decided to halve their rations, doubling the amount of time that they could hold out. The defence was led by Delphion, who with the help of three hundred of the most determined democrats, was able to suppress any opposition with in the city, motivate the rest of the population to man the walls, and lead a number of sorties against the besiegers.
Eventually the food ran out, probably in 379 (but possibly in 380). The defenders asked for a truce to send an embassy to Sparta, and stated that they had decided to leave their fate entirely in the hands of the city authorities. This angered Agesilaus, who felt that he was being snubbed, and he made sure that the Spartan authorities decided to leave everything up to him. While the embassy was away Delphion and one companion escaped from the city, clearly realised that their fate would be rather unpleasant.
Agesilaus decided to give the task of reorganising Phlius to a board of 100 men, made up of 50 restored exiles and 50 citizens who had remained in the city. Their first task was to decide who to execute, and they were then to create a new constitution. A Spartan garrison remained in place for six months, presumably to make sure that the board made the right decisions. Agesilaus then disbanded his army, and returned home.
The surrender of Phlius came at about the same time as the surrender of Olynthus, and Spartan power appeared to be at its height. Most of its main rivals had been humbled or were now allies, and even Athens was unwilling to risk provoking the Spartans. This would soon change. In 379 the Thebans expelled the Spartan garrison and the pro-Spartan government, triggering the Theben-Spartan or Boeotian War (379-371 BC). This ended with the battle of Leuctra (371 BC), the first time that the main Spartan army had been defeated in a hoplite battle, and a defeat that triggered the start of a startlingly rapid decline in Spartan power.