During the war, Macedon attempted to gain control over parts of Illyria and Greece, but without success. It is commonly thought that these skirmishes in the east prevented Macedon from aiding the Carthaginian general Hannibal in the war with Rome. The "Peace of Phoenice", a treaty drawn up at Phoenice, in 205 BC, formally ended the war.
Demetrius had been, after the First Illyrian War in 229 BC, ruler of most of coastal Illyria. However, in 219 BC, during the Second Illyrian War he was defeated by the Romans and fled to the court of Philip.
Involved in a war with the Aetolians, Philip learned by messenger of the victory of Hannibal over the Romans, at Lake Trasimene in June of 217 BC. Philip at first showed the letter only to Demetrius. Perhaps seeing a chance to recover his kingdom, Demetrius immediately advised the young king to make peace with the Aetolians, and turn his attentions toward Illyria and Italy. Polybius quotes Demetrius as saying:
Philip was easily persuaded.
At any rate, Philip chose to build lembi. These were the small fast galleys used by the Illyrians. They had a single bank of oars and were able to carry 50 soldiers in addition to the rowers. With these, Philip could hope to avoid or evade the Roman fleet, preoccupied as he hoped it would be with Hannibal, and based, as it was, at Lilybaeum in western Sicily.
Philip had in the meantime expanded his territories west along the Apsus and Genusus river valleys, right up to the borders of Illyria. Philip's plan was it seems, to first take the Illyrian coasts, conquer the area between the coasts and Macedon, and use the new land link to provide a rapid route for reinforcements across the narrow straits to Italy.
At the beginning of summer, Philip and his fleet left Macedon, sailed through the Euripus Strait, between the island of Euboea and Boeotia on the Greek mainland, and then rounded Cape Malea, before dropping anchor off the Islands of Cephalenia and Leucas, to await word of the location of the Roman fleet. Informed that it was still at Lilybaeum, he sailed north to Apollonia in Illyria.
However, as the Macedonia fleet neared the island of Sazan, Philip heard a report that some Roman quinqueremes had been seen headed for Apollonia. Convinced that the entire Roman fleet was sailing to apprehend him, Philip ordered an immediate return to Cephalenia. Polybius speaks of "panic" and "disorder" to describe the fleet's hasty retreat, and says that in fact the Romans had sent only a squadron of ten ships, and that because of "inconsiderate alarm", Philip had missed his best chance to achieve his aims in Illyria, returning to Macedon, "without loss indeed, but with considerable dishonour".
After hearing of Rome's disastrous defeat at the hands of Hannibal at Cannae in 216 BC, Philip sent ambassadors to Hannibal's camp in Italy to negotiate an alliance. There they concluded in the summer of 215 BC a treaty, the text of which is given by Polybius. In it they pledge in general terms, mutual support and defense and to be enemies to each other's enemies (excepting current allies). Specifically they promise support against Rome, and that Hannibal shall have the right to make peace with Rome, but that any peace would include Philip and that Rome would be forced to give up control of Corcyra, Apollonia, Epidamnus, Pharos, Dimale, Parthini, and Atintania and "to restore to Demetrius of Pharos all those of his friends now in the dominion of Rome.
The treaty as set down by Polybius, makes no mention of an invasion of Italy by Philip, the debacle at Sazan perhaps having soured Philip on such a venture — something which in any case Hannibal may not have desired.
On their way back to Macedon, Philip's emissaries along with emissaries from Hannibal were captured, by Publius Valerius Flaccus, commander of the Roman fleet patrolling the southern Apulian coast. A letter from Hannibal to Philip, and the terms of their agreement were discovered.
Philip's alliance with Carthage caused immediate dismay in Rome, hard-pressed as they already were. An additional twenty-five warships were at once outfitted and sent to join Flaccus' fleet of twenty-five warships already at Tarentum, with orders to guard the Italian Adriatic coast, and to try to determine Philip's intent and if necessary cross over to Macedonia, keeping Philip confined there.
Meanwhile the Romans had moved the fleet from Tarentum to Brundisium to continue the watch on the movements of Philip and a legion had been sent in support, all under the command of the Roman propraetor Marcus Valerius Laevinus. Upon receiving word from Oricum of events in Illyria, Laevinus crossed over with his fleet and army. Landing at Oricum, Laevinus was able to retake the town with little fighting.
In the account given by Livy, Laevinus, hearing that Apollonia was under siege, sent 2000 men under the command of Quintus Naevius Crista, to the mouth of the river. Avoiding Philip's army, Crista was able to enter the city by night unobserved. The following night, catching Philip's forces by surprise, he attacked and routed their camp. Philip, escaping to his ships in the river, made his way over the mountains and back to Macedonia, having burned his fleet and left many thousands of his men that had died or been taken prisoner, along with all of his armies' possessions, behind. Laevinus and his fleet wintered at Oricum.
Twice thwarted in attempts at invasion of Illyria by sea, and now constrained by Laevinus' fleet in the Adriatic, Philip spent the next two years 213–212 BC making advances in Illyria by land. Keeping clear of the coast, he took the inland towns of Atintania, and Dimale, and subdued the Greek tribe of the Dassaretae and the Illyrian Parthini and at least the southern Ardiaei.
He was finally able to gain access to the Adriatic by capturing Lissus and its seemingly impregnable citadel, after which the surrounding territories surrendered. Perhaps the capture of Lissus rekindled in Philip hopes of an Italian invasion. However the loss of his fleet, meant that Philip would be dependent on Carthage for passage to and from Italy, making the prospect of invasion considerably less appealing.
Laevinus had begun exploring the possibility of an alliance with the Aetolian League, as early as 212 BC. The Aetolians, war weary, had made peace with Philip at Naupactus in 217 BC. However five years later the pendulum had swung in the other direction, the war faction was on the ascendency, and the Aetolians were once again ready to consider taking up arms against their traditional enemy of Macedon.
In 211 BC an Aetolian assembly was convened for discussions with Rome. Laevinus pointed out the recent capture of Syracuse and Capua in the war against Carthage, as evidence of Rome's rising fortunes, and offered to ally with them against the Macedonians. A treaty was signed. The Aetolians would conduct operations on land, the Romans at sea. Rome would keep any slaves and other booty taken and Aetolia would receive control of any territory acquired. Another provision of the treaty allowed for the inclusion of certain allies of the League: Elis, Sparta, Messenia and Attalus I of Pergamon, as well as two Roman clients, the Illyrians Pleuratus and Scerdilaidas.
Upon hearing of the Roman alliance with Aetolia, Philip's first action was to secure his northern borders. He conducted raids in Illyria at Oricum and Apollonia and seized the frontier town of Sintia in Dardania or perhaps Paionia. He marched rapidly south through Pelagonia, Lyncestis and Bottiaea and on to Tempe which he garrisoned with 4000 men. He turned north again into Thrace, attacking the Maedi and their chief city Iamphorynna before returning to Macedon.
No sooner had Philip arrived there, when he received an urgent plea for help from his ally the Acarnanians. Scopas the Aetolian strategos (general), had mobilized the Aetolians army and was preparing to invade Acarnania. Desperate and overmatched, but determined to resist, the Arcanians sent their women, children and old men to seek refuge in Epirus, and the rest marched to the frontier, having sworn an oath to fight to the death, "invoking a terrible curse" upon any who were forsworn. Hearing of the Acarnanians' grim determination, the Aetolians hesitated. Then learning of Philip's approach they finally abandoned their invasion. After which Philip retired to Pella for the winter.
Although there was some fear of Rome and concern with her methods, the coalition arrayed against Philip continued to grow. As allowed for by the treaty, Pergamon, Elis and Messenia, followed by Sparta, all agreed to join the alliance against Macedon. The Roman fleet together with the Pergamon fleet controlled the sea, and Macedon and her allies were threatened on land by the rest of the coalition. The Roman strategy of encumbering Philip with a war among Greeks in Greece was succeeding, so much so that when Laevinus went to Rome to take up his consulship, he was able to report that the legion deployed against Philip could be safely withdrawn.
However the Eleans, Messenians and Spartans remained passive throughout 210 BC, and Philip continued to make advances. He invested and took Echinus, using extensive siegeworks, having beaten back an attempt to relieve the town by the Aetolian strategos Dorimachus and the Roman fleet, now commanded by the proconsul Publius Sulpicius Galba. Moving west Philip probably also took Phalara the port city of Lamia, in the Maliac Gulf. Sulpicius and Dorimachus took Aegina, an island in the Saronic Gulf, which the Aetolians sold to Attalus, the Pergamene king, for thirty talents, and which he was to use as his base of operations against Macedon in the Aegean Sea.
In the spring of 209 BC, Philip received requests for help from his ally the Achaean League in the Peloponnesus who were being attacked by Sparta and the Aetolians. He also heard that Attalus, had been elected one of the two supreme commanders of the Aetolian League, and rumors that he intended to crossover the Aegean from Asia Minor. Philip marched south into Greece. At Lamia he was met by an Aetolian force, supported by Roman and Pergamene auxiliaries, under the command of the Attalus' colleague as strategos, the Aetolian Phyrrhias. Philip won two battles at Lamia, inflicting heavy casualties on Phyrrhias' troops. The Aetolians and their allies were forced to retreat inside the city walls, where they remained, unwilling anymore to give battle.
Philip marched to Chalcis in Euboea, which he garrisoned to block Attalus' landing there, then continued on to Aegium for the conference. The conference was interrupted by a report that Attalus had arrived at Aegina, and the Roman fleet was at Naupactus. The Aetolian representatives, emboldened by these events, at once demanded that Philip return Pylos to the Messenians, Atintania to Rome and the Ardiaei to Scerdilaidas and Pleuratus. "Indignant", Philip quit the negotiations telling the assembly that they "might bear him witness that whilst he was seeking a basis for peace, the other side were determined to find a pretext for war".
Philip then joined Cycliadas the Achaean general, near Dyme for a joint attack on the city of Elis, the main Aetolian base of operations against Achaea. However, Sulpicius had sailed into Cyllene and reinforced Elis with 4000 Romans. Leading a charge, Philip was thrown from his horse. Fighting on foot Philip became the object of a fierce battle, finally escaping on another horse. The next day Philip captured the stronghold of Phyricus, taking 4000 prisoners and 20,000 animals. Hearing news of Illyrian incursions in the north Philip abandoned Aetolia and returned to Demetrias in Thessaly.
Meanwhile Sulpicius sailed round into the Aegean and joined Attalus on Aegina for the winter. In 208 BC the combined fleet of thirty-five Pergamene and twenty-five Roman ships failed to take Lemnos, but occupied and plundered the countryside of the island of Peparethos (Skopelos), both Macedonian possessions.
Attalus and Sulpicius then attended a meeting in Heraclea Trachinia of the Council of the Aetolians which included representatives from Egypt and Rhodes, who were continuing to try to arrange a peace. Learning of the conference and the presence of Attalus, Philip marched rapidly south in an attempt to break up the conference and catch the enemy leaders, but arrived too late.
Surrounded by foes, Philip was forced to adopt a defensive policy. He distributed his commanders and forces and set up a system of beacon fires at various high places to communicate instantly any enemy movements.
After leaving Heraclea, Attalus and Sulpicius sacked both Oreus, on the northern coast of Euboea and Opus, the chief city of eastern Locris. The spoils from Oreus had been reserved for Sulpicius, who returned there, while Attalus stayed to collect the spoils from Opus. However, with their forces divided, Philip, alerted by signal fire, attacked and took Opus. Attalus caught by surprise was barely able to escape to his ships.
The neutral trading powers were ever trying to arrange a peace. At Elateia, Philip had met with the same would be peacemakers from Egypt and Rhodes, who had been at the meeting in Heraclea, and again in the spring of 207 BC, but to no avail. Representatives of Egypt, Rhodes, Byzantium, Chios, Mytilene and perhaps Athens also met again with the Aetolians that spring. The war was going Philip's way, but the Aetolians, although now abandoned by both Pergamon and Rome, were not yet ready to make peace on Philip's terms. However, after another season's fighting, they finally were. In 206 BC, the Aetolians, without Rome's consent, sued for a separate peace on conditions imposed by Philip.
The following spring the Romans sent the censor Publius Sempronius Tuditanus with 35 ships and 11,000 men to Dyrrachium in Illiria, where he incited the Parthini to revolt and laid siege to Dimale. However when Philip arrived Sempronius broke off the siege and withdrew inside the walls of Apollonia. Sempronius tried unsuccessfully to entice the Aetolians to break their peace with Philip. With no more allies in Greece, but having achieved their objective of preventing Philip from aiding Hannibal, the Romans were ready to make peace. A treaty was drawn up at Phoenice in 205 BC, the so-called "Peace of Phoenice", formally ended the First Macedonian War.