The Achaean League was reformed early in the 3rd century BC, and soon expanded beyond its Achaean heartland. It was first joined by the city of Sicyon, which provided it with its first great leader, Aratus of Sicyon. The League soon grew to control much of the Peloponnesus, considerably weakening the Macedonian hold on the area, but soon it ran into difficulties with the revived Sparta of Cleomenes III. Aratus was forced to call in the aid of the Macedonian King, Antigonus Doson, to defeat Cleomenes in Sellasia. Antigonus re-established Macedonian control over much of the region.
In 220 BC, the Achaean League entered into a war against the Aetolian League, which was called "the second Allied War". The young king of Macedon Philip V sided with the Achaeans and called for a Panhellenic conference in Corinth, where the Aetolian aggression was condemned.
After Aratus's death, however, the League was able to reap much of the benefits of Macedon's defeat by Rome in 197 BC. Under the leadership of Philopoemen, the League was able to finally defeat a heavily weakened Sparta and take control of the entire Peloponnesus.
The League's dominance was not to last long, however. During the Third Macedonian War (171-168 BC), the League flirted with the idea of an alliance with Perseus, and the Romans punished it by taking several hostages to ensure good behavior, including Polybius, the Hellenistic historian who wrote about the rise of the Roman Empire. In 146 BC, the league erupted into open revolt against Roman domination. The Romans under Lucius Mummius defeated the Achaeans, razed Corinth and dissolved the league. Lucius Mummius received the cognomenAchaicus ("conqueror of Achaea") for his role.
The Achaian army was an army of the traditional hoplite type. From the 270’s onwards however, much like the rest of Greece, the emergence of the Gallic shield known as the Thureos was incorporated into Greek warfare and a new type of troop was developed. Reforming their troops into Thureophoroi, the Achaian army was now composed of light troops. The Thureophoroi were a mixture of an evolved peltasts and a light hoplite, carrying the thureos shield, a thrusting spear and javelins. Plutarch tells us of how they could be effective at a distance, but in close combat the narrow Thureos shield disadvantaged them. He also describes how they would form a formation of sorts, but it would be ineffective, as it would not have inter-locked shields or a ‘leveled line of spears’ (Plut. Philo. 9). Aratus, one of the major Achaian strategoi and statesmen was known for his use of light forces for irregular operations, a type of warfare suited to the Thureophoroi but not suited to operations in the open field. The League in 217 decided to maintain a standing force of 8,000 mercenary foot and 500 cavalry, added to a picked citizen force of 3,000 infantry and 300 cavalry. The citizen infantry would have been armed as Thureophoroi, apart from the citizen light troops who would have been archers and slingers etc. This picked citizen force may well have existed before these so-called reforms, at least on an official basis, as we know of a similar elite force of the same size Sellasia in 222. However, it was the Achaian general Philopoemen in 208 who changed the Achaian fighting style and weaponry. 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’ (Plut. Philo. 9). These ‘reforms’ were not necessarily new to some of the constituent cities of the League, the city of Megalopolis had been given bronze shields and armed in the Macedonian fashion by Antigonus Doson for the Sellasia campaign many years before. As well as reforming and re-organizing the infantry, Philopoemen also did this with the citizen cavalry. The cavalry was recruited, much like in other Greek states, from the rich and noble classes. 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.
