the next sixteen years the Achaian koinon under the leadership of Aratos of Sikyon continued to pursue a vigorously anti-Antigonid politics and to welcome new communities into its ranks, while Aitolian ambitions set the two confederacies at odds until they joined forces for over a decade to battle Antigonid power. The Boiotian koinon, comparatively limited in its growth potential by the vigor of its western and southern neighbors as well as by the security of Macedonian control over nearby Athens, was more often than not caught in the middle of larger military and political contests. But its rich epigraphic record belies this impression of unimportance, at least to the history of political cooperation. This was, finally, the last period in which the koina of mainland Greece operated entirely free of the shadow of Rome, the arrival of which radically changed the dynamics of cooperation.
In 245 the Achaians under the leadership of Aratos of Sikyon adopted a policy of hostility toward the Aitolians, and in preparation for war they made an alliance with the Boiotians. The allies invaded Aitolia and Ozolian Lokris, now a part of the Aitolian koinon, but the Aitolians retaliated by invading Boiotia and destroying the Boiotian army at the old battle site of Chaironeia.75 The Achaians arrived too late to help, and the Boiotians now joined the Aitolian koinon.76 Refocusing, after this defeat, on Achaian expansion in the Peloponnese, Aratos seized Corinth in 243 and immediately brought it into the koinon.77 He secured his prize by crossing the isthmus and bringing Megara, the near neighbor of now pro-Aitolian Boiotia, into the koinon, and protected Corinth’s southeastern exposure by winning over Troizen and Epidauros, on the Saronic Gulf.78 The introduction of four large poleis, including the vital stronghold of Acrocorinth, dramatically increased the Achaians’ strategic power.
Sometime in the mid-240s, whether in response to the tremendous growth of the Achaian koinon in 243 or slightly before it, the Aitolians and Antigonos Gonatas reportedly “made an agreement to partition the Achaian ethnos between them.”79 The Achaians now faced pressure from both enemies on the periphery of their territory. The Aitolians, through the support of their Elian kin, gained influence without formal control in the western Peloponnese in these years.80 They and the Elians may also have been involved in the tyrannical coup of one Lydiades at Arkadian Megalopolis.81 But the Aitolians were still focused on expanding their koinon in mainland Greece. Polybios twice mentions, in connection with the Aitolian-Antigonid agreement to partition Achaia, an agreement made between the Aitolians and Alexander, the king of Epeiros, to partition Akarnania.82 This move was a direct violation of the earlier treaty between the Aitolians and Akarnanians (T57), which in the larger history of Aitolian-Akarnanian relations appears to have been truly exceptional. Antigonos may have countenanced such an aggressive strategy of territorial acquisition by both his Epeirote neighbors and the Aitolians in order to palliate any lingering Aitolian concerns about the benefits accruing to them from the agreement to attack the Achaians. It had thus far proved impossible for the Aitolians to exert anything but influence in the Peloponnese, and that only by way of their Elian kin; so what material benefits would they gain?83 The sudden abandonment of the treaty with Akarnania, absent any provocation, may also reflect a change in Aitolian political leadership.84 The process by which the acquisition of southern Akarnania was accomplished by the Aitolians is lost to us, but it is clear that by the 230s the important cities of Stratos and Oiniadai, along with lesser settlements like Matropolis, were part of the Aitolian koinon (T59).85 The war between the Achaians and Aitolians that should have resulted from the purported Aitolian-Antigonid agreement never materialized. The Aitolians invaded the Peloponnese, but the Achaians refused to meet them in battle, defended Pellene from their attack, and by spring 240 succeeded in winning over Arkadian Kynaitha as a member of the koinon.86 The Achaians made peace with the Aitolians and concluded a truce with Gonatas.87
The 230s were years of growth for the Achaian koinon under the continued leadership of Aratos, and they brought expansion, albeit on a smaller scale, for the Aitolians as well. This growth was, however, marked in both cases by hostilities with the new Macedonian king, Demetrios II. The Achaians focused on expanding their territory in the directions they had already staked out: the Argolid, which would complete Achaian control over the entire northern Peloponnese, and Arkadia, which would allow for southerly expansion. Peace with the Aitolians meant, at least ostensibly, that the Elians would be friendly, and there was no outright rupture in Achaian-Spartan relations. Opposition to the Achaians remained, however, in the presence of a number of local tyrants, most of them supported, if not installed, by Gonatas and now friendly to Demetrios. Chief among them was Lydiades of Megalopolis, who also controlled Alipheira, together perhaps with Heraia, Telphousa, and Kleitor, thereby extending his territory almost as far as Kynaitha.88 This made conflict with the Achaians almost inevitable. Argos, too, was in the hands of a local Macedonian-supported tyrant, and Aratos made his first (unsuccessful) attempt on the city probably around 240.89
The peace between the Achaians and Aitolians was converted, around 239, into a formal alliance.90 The change may have been motivated by a new alliance between Epeiros and Macedonia that was at least potentially anti-Aitolian.91 No explicit statement of the terms of the alliance survives, but a careful reading of the slim record of Achaian-Aitolian cooperation over the next two decades suggests that there were at least two: first, that the allies would render military assistance to one another, and jointly to friends who sought it out; and second, that allied troops would be paid according to some predetermined wage.92 It was probably in this year that the so-called Demetrian War broke out between the two sets of allies, but our sources are exceptionally scarce.93
Over the next decade the Achaian koinon grew vigorously, while our limited evidence suggests that the Aitolians lost ground in central Greece and struggled to gain it in Akarnania. In the Peloponnese, Aratos continued to lead the Achaians on attacks aimed at ousting the tyrants of Argos and attaching the city to the koinon, won over Kleonai, which now became a member, and accepted the adherence of Megalopolis, where Lydiades surrendered his tyranny in 235 and brought his city into the Achaian koinon.94 In the previous year the Achaians had won Arkadian Heraia, which suggests that they had already won over Kleitor and Telphousa.95 The admission of Megalopolis brought the Achaians further south into Arkadia, integrating a large and powerful city but one that had pursued, since its foundation in the 360s, a policy of implacable hostility toward the Spartans. It was inevitable that Megalopolis’s membership in the koinon would embroil the Achaians in those same hostilities, particularly in the years 234–230, during which Lydiades was elected stratēgos three times, alternating in years of service with Aratos, and the ambitious young king Kleomenes III had recently come to power in Sparta.96 Eastern Arkadia followed quickly, with Orchomenos, Mantineia, Tegea, and probably Kaphyai joining around 233.97
In central Greece, meanwhile, Demetrios II was attacking Aitolian interests. Around 234 he invaded Boiotia, which had been an Aitolian ally since the disastrous battle of Chaironeia in 245, and separated the Boiotian koinon from Aitolia.98 It was probably during the same expedition that he wrested Opountian Lokris from the Aitolians and attached it to the now-allied Boiotian koinon, where it remained until circa 228.99 The Aitolians may, however, have gained some ground against Demetrios in northeastern central Greece.100 The Aitolian alliance with the Achaians prompted Demetrios to invaded the Peloponnese in 233. The Achaians under the leadership of Aratos suffered a serious defeat at Phylakia, just south of Tegea.101 It may have been as a result of this defeat that the Achaians became willing to cede their newest Arkadian members—Orchomenos, Mantineia, Tegea, and Kaphyai—to the Aitolians, who may have offered to help the Achaians stabilize the area.102
Whatever the nature of their gains in Arkadia, the Aitolians continued to wage war against Demetrios’s interests and to pursue their own agenda of westward expansion in mainland Greece. In 232 the Aiakid dynasty that had ruled Epeiros for centuries came to an end, and the inhabitants of both Epeiros itself and the Epeirote part of Akarnania chose to form independent koina by which they planned to govern themselves.103 The Aitolians seized the moment to attack western Akarnania, hoping to gain access to the Adriatic. In the autumn of 231, they laid siege to Medion, a small Akarnanian polis south of the Ambrakian Gulf, having been “unable