The jealousy which the Ætolians had long entertained of the Achæan states, was increased by the importance which they had assumed from their alliance with Macedon. No sooner were they relieved from the dread of Antigonus, than they ravaged the Achæan coast, and committed depredations on all the neighbouring countries. Aratus having opposed to them the Achæan forces in vain, invoked and obtained the aid of the king of Macedon. Philip promised that as soon as he should have settled the affairs of his own kingdom, he would repair to Corinth, in order to meet the convention of the states in alliance with Achaia, that he might have an opportunity of settling with them a plan of future operations. In the meantime, the Ætolians, making a fresh irruption into Peloponnesus, sacked Cynætha, a city of Arcadia, put most of the inhabitants to the sword, and laid the place in ruins. The convention of the Achæan confederates, now assembled at Corinth, unanimously agreed that unless the Ætolians should make reparation, war should be declared against them, and the direction of it committed to the king of Macedon. Hence the origin of the Social War, so called from the association entered into by the several states engaged against Ætolia.
Philip commenced his operations with the siege of Ambracas, a fortress which commanded an extensive territory, belonging of right to Epirus, but now in the hands of the Ætolians. Having reduced this fortress, he restored it to the Epirots, and prepared to carry the war into Ætolia. The Ætolian spirit was not daunted either by the loss of Ambracas or the threats of Philip. They invaded Macedon, and made incursions into Achaia, which they reduced to the greatest distress. The mercenaries in the Achæan service had mutinied for want of pay; the Peloponnesian confederates became spiritless or disaffected; even the Messenians, in whose cause chiefly Achaia, had, at the beginning, taken up arms, were afraid to act against the Ætolians: whilst the Spartans, notwithstanding their engagements, at the late conventions, to Achaia, had now massacred, or sent into exile, all such of their own citizens as were in the interest of the Achæans, and openly declared against them. For the Spartans, amidst their greatest humiliation, had ever been impatient of the domination of Achaia, to which the haughtiness of that republic had, in all probability, very much contributed.
[216-208 B.C.]
A year had elapsed since the alliance had been formed against Achaia, when Philip of Macedon, in the depth of winter, set out with the utmost secrecy to Corinth, where a part of his forces were stationed. He surprised a party of Eleans, who had gone forth to ravage the Sicyonian territories, and reduced Psophis, a stronghold within the confines of Arcadia, of which the Eleans had taken possession. He plundered Elis, one of the finest regions in Greece, in respect to cultivation, and rich in every kind of rural wealth. He next subdued under his power Triphylia, a district of Peloponnesus to the southward of Elis, and wrested the Ætolian yoke from the necks of the Messenians. Philip made a temperate use of all his victories. He granted peace to all who sued for it; and the whole of his conduct seemed to be directed by the same generous motives which had formerly directed that of Antigonus. But in the midst of these fair appearances, Philip began to manifest latent seeds of ambition. He restrained the pride and power of his ministers, who had been appointed to their offices by his predecessor Antigonus; and supported Eperatus in the election of general of Achaia, in opposition to Aratus. In order to counterbalance this unpopular measure, and to strengthen himself in the affections of the Achæan people, he besieged Teichos, and having taken that fortress, restored it to the Achæans, to whom it belonged. He also made an inroad into Elis, and presented the Dymeans and the cities in the neighbourhood with all the plunder. He now imagined that the wealth and vigour of the Achæan republic were at his disposal; but the new general had not provided any magazines, and the treasury was exhausted. Philip now affected to place great confidence in Aratus. By the advice of this statesman, he made an attempt on the island of Cephallenia, an island in the Ionian Sea, near the coast of Peloponnesus, and the great resort of the Ætolian pirates. His attempt, after it had been carried on almost to success, was baffled by the treachery of his ministers.