Celtic tribesmen (Gauls) under Brennus invade Macedonia and crush a Macedonian army led by Ptolemy Keraunos who is killed in the battle. At the narrow pass of Thermopylae, on the east coast of central Greece, Brennus' forces suffer heavy losses while trying to break through the Greek defence comprising the Phocians and the Aetolians. Eventually Brennus finds a way around the pass but the Greeks escape by sea.
Brennus pushes onto Delphi which he sacks, but he is wounded and his forces defeated in a battle near Delphi by the Macedonian general Sosthenes. Despite the losses at Delphi, Brennus' forces then penetrate into Epirus and sack the treasures of the temples of Zeus at Dodona and Olympia.
Facing determined Greek resistance, Brennus withdraws to Macedonia, where he dies from his wounds. Without him, the Gauls in his army divide. Some of them cross the Bosporus and settle in a part of Asia Minor that will come to be called Galatia, while some settle in Thrace, founding a short-lived city-state named Tylis.
With the death of Ptolemy Keraunos, the previous King of Macedonia, Antipater II becomes king again. However, his new reign lasts only few months before he is killed by his cousin Sosthenes who becomes the new King of Macedonia.
The Phocians are readmitted into the Amphictyonic League after they have joined in the defence of Delphi against the Gauls.
The Carthaginians and the Romans agree to support each other against a common foe. The Carthaginians give Rome money and ships in their fight against Pyrrhus, the king of Epirus.
Pyrrhus realises that he cannot capture Rome and suggests peace terms to the Romans. Pyrrhus send his chief advisor, Cineas, to Rome to negotiate a peace. Cineas demands that the Romans halt their aggression against the Greeks of southern Italy and restore the lands the Romans have taken from the Bruttii, the Apulians, and the Samnites. The Romans reject his demands, largely at the instigation of the former Roman censor, Appius Claudius Caecus.
In renewed fighting, Pyrrhus of Epirus, leading the combined Tarantine, Oscan, Samnite, and Epiriotic forces, wins a 'Pyrrhic victory' against the Romans led by consul Publius Decius Mus at the Battle of Asculum, called such because his victory comes at a great cost to his own forces. Pyrrhus is reported to have said afterwards, "One more victory against the Romans and we shall be utterly ruined!" Disheartened, Pyrrhus retires to Tarentum and sends Cineas to make renewed peace overtures to Rome. These talks are inconclusive.
