Tensions rose dramatically in 148 BC, when Achaea defeated and finally subjugated Sparta; in the aftermath of this, Rome tried to cow the League into halting its expansionist ambitions, but a failure of diplomacy between the two sides led to war.

The Roman Republic had developed close ties to the Achaean League through similar religious and military beliefs and a cooperation in the previous Macedonian Wars.

Roman foreign policy in the Greek east in the period following the Third Macedonian War had also become increasingly in favour of micromanagement and the forced breaking-up of large entities, seen by the regionalisation of Macedon by the general Lucius Mummius Achaicus and the Senate's mission to the magistrate Gallus, upon the application of the town Pleuron to leave the Achaean League, to sever as many cities from it as possible; Pausanias writes that Gallus "behaved towards the Greek race with great arrogance, both in word and deed".

The Acheans swiftly defeated Sparta, but the League's strategos of 149, Damocritus, decided not to press the offensive further, either due to Roman pressure or a policy of pacifism.

The strategos of 148, Diaeus, was elected on a platform of aggression and League unity, and hence swiftly pressed home the attack, and subjugated Sparta by the end of the year.

Orestes tried to announce the forced reduction of the Achaean League to its original, narrow grouping - effectively crippling it and ending its territorial ambitions once and for all.

[11] A Roman effort at restoring peace, led by Orestes' former co-consul Sextus Julius Caesar, went badly, and the Achaeans, outraged at Rome's actions, and whipping up populist sentiment, declared war on Sparta, electing Critolaos as strategos of the league.

[16] However, many elements of the League, especially Corinth, rallied around Diaeus, electing him as strategos to replace Critolaos and resolving to continue the war, with harsh levies and confiscations of property and wealth.

[19][20] The surviving Achaean troops could have been organized to defend the city, but Diaeus abandoned them and fled to Megalopolis, where he committed suicide after killing his wife.

[21] Demoralized, the Achaean troops and most Corinthians fled the city, leaving it defenseless, allowing the Romans to secure it, though only three days after the battle, as Mummius feared an ambush.

This apparently needless display of cruelty in Corinth, is explained by Mommsen as due to the instructions of the Senate, prompted by the mercantile party, which was eager to dispel a dangerous commercial rival.

[28] Mummius was extremely ignorant in matters of art - when transporting priceless statues and paintings to Italy, he gave orders that the contractors should be warned that if they lost them, they would have to replace them by new ones.

Pergamon, the only significant remaining power in the Aegean, was generally pro-Roman, and its last king, Attalus III, bequeathed it to Rome through his will upon his death in 133 BC.