An army of Gauls under Brennusinvade Greece. A section of the army, commanded by Bolgios, crushes 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 on to Delphi where he is defeated and forced to retreat, after which he dies of wounds sustained in the battle. His army falls back to the river Spercheios where it is routed by Thessalians and Malians. Some of the survivors settle in a part of Asia Minor that will eventually 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 realizes that he cannot capture Rome and suggests peace terms to the Romans. Pyrrhus sends 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 Greek 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.
After their defeats in Greece, the Gauls move into Asia Minor. The Seleucid king Antiochus wins a major battle over the Gauls leading to his being given the title of Soter (Greek for "saviour"). The Gauls settle down to become the "Galatians" and are paid 2,000 talents annually by the Seleucid kings to keep the peace.
Antigonus concludes a peace with Antiochus who surrenders his claim to Macedonia. Thereafter Antigonus II's foreign policy is marked by friendship with the Seleucids.
Nicomedes I becomes the first ruler of Bithynia to assume the title of king. He founds the city of Nicomedia, which soon rises to great prosperity.
Chu's heartland in the modern Hubei province is overrun by the powerful state of Qin from the west under Bai Qi's leadership. The Chu government moves to the east in various temporary capitals until settling in Shouchun in 241 BC.
The Egyptian King Ptolemy II's first wife, Arsinoe I (daughter of the late King Lysimachus of Thrace) is accused, probably at instigation of Ptolemy II's sister (who also has the name Arsinoe), of plotting his murder and is exiled by the King. Arsinoe then marries her own brother, a customary practice in Egypt, but scandalous to the Greeks. The suffix "Philadelphoi" ("Brother-Loving") consequently is added to the names of King Ptolemy II and Queen Arsinoe II. The former queen, Arsinoe I, is banished to Coptos, a city of Upper Egypt near the Wadi Hammamat, while her rival adopts her children.
The first of the Syrian Wars starts between Egypt's Ptolemy II and Seleucid emperor Antiochus I Soter. The Egyptians invade northern Syria, but Antiochus defeats and repels his opponent's army.
Pyrrhus negotiates with the Carthaginians to end the fighting between them in Sicily. The Carthaginians are inclined to come to terms with Pyrrhus, but he demands that Carthage abandon all of Sicily and make the Libyan Sea the boundary between Carthage and the Greeks. Meanwhile, he begins to display despotic behaviour towards the Sicilian Greeks and soon Sicilian opinion moves against him. Therefore, fearing that his successes in Sicily may lead him to become the despot of their country, the Syracusans ask Pyrrhus to leave Sicily. He does so, and returns to the Italian mainland, noting that he expects Sicily to be a "fair wrestling ring" for Carthage and Rome.
Following the departure of Pyrrhus from Sicily, the Syracusan army and the city's citizens appoint Hiero II as the commander of their slaves. He strengthens his position by marrying the daughter of Leptines, the city's leading citizen.
Magas of Cyrene marries Apama, the daughter of Antiochus and uses his marital alliance to foment a pact to invade Egypt. He opens hostilities against his half brother Ptolemy II, by declaring his province of Cyrenaica to be independent and then attacks Egypt from the west as Antiochus I takes the Egyptian controlled areas in coastal Syria and southern Anatolia, after which he attacks Palestine.
Magas has to stop his advance against Ptolemy II due to an internal revolt by the Libyan Marmaridae nomads.
Pyrrhus' departure from southern Italy three years earlier leads to the Samnites finally being conquered by the Romans. With the surrender of Tarentum, the cities of Magna Graecia in southern Italy come under Roman influence and become Roman allies. Rome now effectively dominates all of the Italian peninsula.
Cleonymus, a Spartan of royal blood who has been outcast by his fellow Spartans, asks the King of Macedonia and Epirus, Pyrrhus, to attack Sparta and place him in power. Pyrrhus agrees to the plan, but intends to win control of the Peloponnese for himself. As a large part of the Spartan army led by king Areus I is in Crete at the time, Pyrrhus has great hopes of taking the city easily, but the citizens organise stout resistance, allowing one of Antigonus II's commanders, Aminias the Phocian, to reach the city with a force of mercenaries from Corinth. Soon after this, the Spartan king, Areus, returns from Crete with 2,000 men. These reinforcements stiffen Spartan resistance and Pyrrhus, finding that he is losing men to desertion every day, breaks off the attack and starts to plunder the country.
As they plunder the countryside, Pyrrhus and his troops move onto Argos. Entering the city with his army by stealth, Pyrrhus finds himself caught in a confused battle with the Argives (who are supported by Antigonus' forces) in the narrow city streets. During the confusion an old woman watching from a rooftop throws a roof tile at Pyrrhus which stuns him, allowing an Argive soldier to kill him.
Following his death in Argos, Pyrrhus is succeeded as king of Epirus by his son Alexander II while Antigonus II Gonatas regains his Macedonian throne which he has lost to Pyrrhus two years earlier.
With the restoration of the territories captured by Pyrrhus, and with grateful allies in Sparta and Argos, and garrisons in Corinth and other Greek key cities, Antigonus II securely controls Macedonia and Greece. Antigonus becomes the chief of the Thessalian League and is on good terms with neighbouring Illyria and Thrace. He secures his position in Greece by keeping Macedonian occupation forces in the cities of Corinth, Chalcis on the island of Euboea, and Demetrias in Thessaly, the three "shackles" of Hellas.
Carthage, already in control of Sardinia, southern Spain and Numidia, is ruled by an oligarchy of merchants under two Suffetes or chief magistrates. While Carthage's military commanders are strong, the state relies on mercenaries (including Spanish ones) for its soldiers.