The Achaean League (Ancient Greek: Κοινὸν τῶν Ἀχαιῶν, romanized: Koinon ton Akhaion, lit.

As a rival of Antigonid Macedon and an ally of the Roman Republic, the league played a major role in the expansion of Rome into Greece.

The League represents the most successful attempt by the Greek city-states to develop a form of federalism, which balanced the need for collective action with the desire for local autonomy.

[2][failed verification] Xenophon's Hellenica reports that Achaea underwent a democratic revolution in 367 and the previous ruling class was exiled.

[3] The regional Achaean League was reformed in 281/0 BC by the communities of Dyme, Patrae, Pharae and Tritaea, joined in 275 by Aegium, which controlled the important sanctuary of Zeus Homarios.

In other cities of the Peloponnese, namely Argos, Orchomenus, and Megalopolis, Antigonus had installed friendly rulers who were perceived as tyrants by the Achaeans.

Aratus, whose father had been killed by one, called for the liberation of these cities and secured financial support for the League from Ptolemy II of Egypt, an enemy of the Antigonids.

[6] Antigonus Gonatas finally made peace with the Achaean League in a treaty of 240 BC, ceding the territories that he had lost in Greece.

Aratus was forced to call in the aid of the Macedonian King, Antigonus III Doson, who defeated Cleomenes at Sellasia.

The young king Philip V of Macedon sided with the Achaeans and called for a Panhellenic conference in Corinth, where the Aetolian aggression was condemned.

Under the leadership of Philopoemen, the League was able to finally defeat a heavily weakened Sparta and take control of the entire Peloponnese.

In c. 120 BC Achaeans of cities in the Peloponnese dedicated an honorary inscription to Olympian Zeus, after a military expedition with Gnaeus Domitius against the Galatians in Gallia Transalpina.

Special meetings (synkletoi) had to be called in order for the league to declare war, form an alliance, or receive official communications from the Macedonian king or (later) the Roman Republic.

[13] The league was ostensibly a democracy, but control seems to have consistently rested with a small elite group who monopolised the generalship and other official positions.

From the 270s onwards however, much like the rest of Greece, the emergence of the shield known as the thyreos was incorporated into Greek warfare and a new type of troop was developed.

The thyreophoroi were a mixture of evolved peltasts and light hoplites, carrying the thureos shield, a thrusting spear and javelins.

[16] Aratus also obtained 500 foot and 50 horse each from Taurion and the Messenians for defence of parts of the League open to attack via Laconia.

[17] According to Plutarch, Philopoemen 'persuaded them to adopt long pike and heavy shield instead of spear and buckler, to protect their bodies with helmets and breastplates and greaves, and to practice stationary and steadfast fighting instead of the nimble movements of light-armed troops'.

Philopoemen organized the cavalry in lochoi, which usually in ancient military treatises means 'files', most probably of 8 men, grouped into dilochiai, a formation of double-files of 16 and so forth.