The alliance between Philip V of Macedon and Hannibal Barca in 215 BC was one of the most potentially dangerous diplomatic efforts to stop the Roman expansion in the Adriatic.
Hannibal had already inflicted catastrophic defeats upon Rome in the Second Punic War, culminating in the destruction of a large Roman army at Cannae in 216 BC. Philip, meanwhile, was the ruler of the strongest kingdom in mainland Greece and had long been interested in expanding Macedonian influence across the Adriatic, particularly in Illyria.
The interests of the two leaders appeared complementary. Hannibal needed Rome to remain under pressure, while Philip wanted to destroy or reduce Roman power on the eastern Adriatic and recover territories that Rome had acquired after the Illyrian Wars. Yet the alliance never developed into the coordinated Macedonian-Carthaginian offensive that Rome feared. The Romans responded rapidly, exploited the divisions of the Greek world, allied themselves with the Aetolian League and Pergamum, and kept Philip occupied in Greece and Illyria.
As a result, the alliance became strategically ineffective, even though the discovery of the alliance formation was mostly accidental. Nevertheless, the episode demonstrates one of Rome’s greatest strengths—the ability to fight simultaneously through military force, diplomacy, and the manipulation of alliances.
Philip’s eye on Illyria
Philip’s decision to approach Hannibal must be understood within the wider struggle for control of the Adriatic. Macedon had already clashed with Rome over Illyria. Philip’s ambitions were encouraged by Demetrius of Pharos, an Illyrian ruler who had previously been a Roman ally but had been defeated by Rome and subsequently taken refuge at the Macedonian court. Philip saw an opportunity when Rome became deeply engaged in the war against Carthage. Hannibal’s victories at the Trebia and Lake Trasimene demonstrated that Rome could be defeated, while Cannae seemed to confirm that the Roman military system was approaching collapse.
Roman historian Titus Livius presents Philip’s calculation in dramatic terms. He writes that Philip, observing the struggle between Rome and Carthage, “was rejoiced that a war had arisen between the Romans and the Carthaginians.” After Hannibal’s successive victories, Philip V of Macedon “inclined to fortune, and sent ambassadors to Hannibal.” This interpretation captures the opportunistic nature of Philip’s policy. He was not simply responding to a Carthaginian request for help. He was attempting to exploit Rome’s moment of greatest vulnerability.
The treaty was concluded in 215 BC. Greek historian Polybius preserves what appears to be the authentic text, and it reveals that the alliance was essentially a mutual military and diplomatic agreement. Philip and the Macedonians promised to support Hannibal and the Carthaginians against Rome, while Hannibal promised assistance to Philip. The treaty declared that the two sides would be “friends, close allies, and brethren.” More importantly, Philip was promised the restoration of Macedonian influence over Roman possessions in the eastern Adriatic. If Rome were defeated, Carthage would support Philip’s claims to Corcyra, Apollonia, Epidamnus, Pharos, Dimale, Parthini, and Atintania.
The strategic plan against the Romans: Hannibal and Philip V of Macedon
The alliance had a clear strategic logic. Hannibal was fighting the Roman Republic in Italy, while Philip could attack Roman interests in the Balkans. In theory, Rome would be forced to fight on two distant fronts. If Philip could invade Illyria and perhaps cross the Adriatic into Italy, Roman resources would have to be diverted from Hannibal. Conversely, if Hannibal defeated Rome in Italy, Philip would be able to expand Macedonian power in the Adriatic. Modern historian Dexter Hoyos observes that Cannae “encouraged Philip V, king of Macedon, to ally with Carthage,” with Philip’s objective being to replace Roman influence along the eastern Adriatic with Macedonian influence.
Later Roman traditions sometimes describe a more ambitious plan under which Philip would send a large fleet to Italy and participate directly in the conquest of Rome. Titus Livius provides a version in which Philip was supposedly to cross into Italy with a fleet and fight on land and sea. However, Polybius’ surviving treaty does not contain such a straightforward commitment to an invasion of Italy. Modern scholars have consequently questioned whether the more dramatic version represents later Roman reconstruction rather than the original agreement. Historian Ian Worthington notes that Philip’s alleged intention to invade Italy remains disputed.
This distinction is noteworthy because it explains why the alliance was not necessarily doomed by poor intentions. The principal problem was geography. Philip could not simply march into Italy. He first needed secure ports on the Adriatic, a functioning fleet, and the ability to transport an army across the sea.
The Romans find out and respond
Rome’s first significant achievement came almost by accident. Philip’s ambassadors were returning from Hannibal’s camp when they were intercepted by Roman naval forces. The Romans discovered the existence of the treaty and learned of Philip’s intentions. Titus Livius records the episode in detail: the Macedonian emissaries were intercepted and brought before the Roman authorities although the envoy initially pretended that he was negotiating friendship with Rome. Once Rome understood what had happened, the Senate acted decisively.
The Roman response was not to wait for Philip to invade. Additional ships were equipped and sent to reinforce the Roman naval forces already operating in southern Italy. Their purpose was to protect the Adriatic coast, observe Philip’s movements and, if necessary, carry the war across the sea. The Roman strategy was fundamentally preventive. Modern scholars emphasize that Rome feared the possibility of a Macedonian intervention in Italy and therefore sought to contain Philip V before he could transform the treaty with Hannibal into effective military cooperation.
The most important Roman countermeasure, however, was diplomatic rather than naval. Rome acknowledged that Philip was a powerful land monarch, but that he could be contained if other Greek states were encouraged to resist him. In 211 BC, the Romans concluded an alliance with the Aetolian League. This was a crucial turning point. The Aetolians were traditional enemies of Macedonian expansion and possessed military forces capable of operating effectively in central Greece. Their alliance meant that Philip could not concentrate his entire strength against Roman interests in Illyria.
Rome’s alliance with the Aetolian League neutralized Philip’s advantage on land. Another ally Rome found was Attalus I of Pergamum, who further distracted the Macedonian king. Rome had managed to transform the conflict from a simple Macedonian-Roman confrontation into a wider coalition war.
Philip V of Macedon faces different fronts: Plans with Hannibal go awry
Philip’s strategic position was lost. He had a strong army, but he was surrounded by competing Greek powers and could not afford to ignore threats to his own kingdom. The Aetolians attacked Macedonian interests, while other Greek states became involved in the struggle. Attalus I of Pergamum also entered the anti-Macedonian coalition. Philip consequently had to divide his attention between defending Macedonia, supporting his Greek allies, fighting the Aetolians, and pursuing his ambitions in Illyria.
The Romans had achieved what they needed without defeating Philip in a decisive battle. They did not have to conquer Macedon. Their objective was to prevent Philip V of Macedon from becoming an effective second front for Hannibal. As modern historians assess, the treaty with Hannibal could not materialize in action because Rome’s alliances and Philip’s other commitments prevented him from turning the agreement into meaningful assistance for Carthage.
Rome’s alliances brought the First Macedonian War, which for Philip became a war of containment. Philip was successful to an extent, especially in Illyria, and he captured Lissus, but he never succeeded in achieving the strategic breakthrough that Hannibal needed. His inability to control the Adriatic prevented a major transfer of Macedonian troops to Italy. Meanwhile, Hannibal’s own position deteriorated. Rome recovered from Cannae, rebuilt its armies, and adopted a strategy of refusing to risk another catastrophic pitched battle while systematically recovering territory and weakening Carthaginian allies.
The Aetolian contribution
The Aetolian alliance perfectly suited Roman circumstances. Rome could not devote the enormous armies necessary for a full-scale conquest of Macedon while Hannibal remained in Italy. Instead, it supplied money, ships, and diplomatic support while encouraging local enemies of Philip to do much of the fighting. A modern study of Roman aims in the First Macedonian War stresses that the Roman Senate feared a Macedonian invasion of Italy and that its policy was consequently directed toward keeping Philip confined to his own kingdom.
Philip nevertheless remained committed to Hannibal to some degree even after the alliance had failed to produce a decisive joint campaign. The clearest evidence came after Hannibal returned to Africa. Philip sent approximately four thousand Macedonian troops under Sopater to assist the Carthaginians. These troops fought at the battle of Zama in 202 BC. Thus the alliance did not disappear simply because the First Macedonian War had ended. It had merely failed in the creation of a coordinated Macedonian-Carthaginian offensive capable of overwhelming Rome, which was its most important strategic objective.
The conclusion of the First Macedonian War came with the Peace of Phoenice in 205 BC. Rome accepted Macedonian possession of several territories in Illyria, while Philip effectively withdrew from the immediate struggle with Rome. This was an economical Roman victory. Philip had not been destroyed, but he had been prevented from helping Hannibal in the manner that Rome feared. Dexter Hoyos summarizes the outcome by stating that Rome “threw Philip out of Illyria” and largely eliminated his participation in Hannibal’s war, even though Philip enjoyed victories against the Aetolian League.
A diplomatic victory
Rome came out victorious after ensuring that the Macedon-Carthage alliance never materialized in actual battle. A simultaneous Carthaginian assault in Italy and Macedonian offensive in the Balkans could have stretched Roman resources to a breaking point. Yet, Rome prevented this possibility through a combination of intelligence, naval control, and diplomacy. The interception of the Macedonian ambassadors exposed the alliance. The Roman fleet restricted Philip’s ability to cross the Adriatic. The alliance with the Aetolian League resulted in a powerful local counterweight to Macedon, while Pergamene involvement further distracted Philip. Most importantly, Rome understood that it did not need to defeat Philip outright, but it merely had to keep him too busy to become Hannibal’s effective partner.
Ancient sources reveal the fears and calculations behind the episode, while modern historians help distinguish between the genuine treaty and later traditions related to a planned invasion of Italy. Polybius’ treaty shows a serious commitment to mutual support, while Titus Livius’ narrative emphasizes the dramatic possibility of Macedonian intervention. Modern scholars, however, generally highlight the practical limitations imposed by geography and the success of Roman coalition-building.
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