222 BC

• The Roman consuls Marcus Claudius Marcellus and Gnaeus Cornelius Scipio Calvus march into Insubres territory and besiege Acerrae, an Insubre fortification on the right bank of the River Adda between Cremona and Laus Pompeia. The Insubres are unable to relieve Acerrae because the Romans control all the strategic points around it. Therefore, they hire 30,000 Gaesatae mercenaries, led by Viridomarus (or Britomartus), who march to Clastidium, an important and strategically situated town, owned by the Marici, a Ligurian tribe allied to the Romans, they start a siege hoping that this will force the Romans to lift their siege of Acerrae to come to Clastidium's aid.Polybius, The Histories, 2.19-20
• The Romans split their forces. Marcus Claudius Marcellus heads for Clastidium while his colleague Gnaeus Cornelius Scipio Calvus continues the siege of Acerrae. At the Battle of Clastidium Marcellus defeats the Gaesatae. Marcellus personally slays Viridomarus. This victory removes the Gallic threat to Rome. Marcellus is awarded the spolia opima for the third and last time in Roman history.Polybius, The Histories, 2.19; Plutarch, Marcellus, 6-7.
• After taking Acerrae Gnaeus Cornelius Scipio Calvus marches on Mediolanum (modern Milan), another stronghold of the Insubres, which also falls into Roman hands, upon which the chieftains of the Insubres lose all hope and surrendered unconditionally. Thus the Romans succeeded in conquering the largest independent Celtic tribe in Italy and firmly established their hegemony over the Po Valley, then the most productive agricultural region in the peninsula.Polybius, The Histories, II 17,4-5 and 20.

• Battle of Sellasia: Cleomenes III of Sparta is defeated by Antigonus III of Macedon and his allies (the Achaean League, the Boeotian League, the Acarnanians, Epirus and the Illyrians under the command of Demetrius of Pharos) near Sellasia (north of Sparta) and flees to Egypt under the protection of King Ptolemy III. Antigonus III's forces occupy Sparta, which is the first time this city has ever been occupied.
• Almost all of Greece falls under Macedonian suzerainty after Antigonus III re-establishes the Hellenic Alliance as a confederacy of leagues, with himself as president.

• Seleucid forces under their general Achaeus succeed in winning back all the Seleucid domains in Asia Minor (lost six years earlier to Pergamum).
• Mithridates II of Pontus gives his daughter Laodice in marriage to the Seleucid king Antiochus III. Another of his daughters, also named Laodice, is married about the same time to general Achaeus.

• The Qin general Wang Jian conquers Wuyue, forcing the capitulation of its ruler. The conquered region becomes the province of Kuaiji. This campaign completes the subjugation of the lands formerly held by the State of Chu, and it also serves as a precursor for the Qin campaign against the Yue tribes.
• The Qin generals Wang Ben and Li Xin conquer Liaodong, thereby completing the subjugation of Yan.
• Wang Ben conquers Dai, thereby completing the subjugation of Zhao.{{cite book|first=Sima|last= Qian|title=Records of the Grand Historian, Section: The First Emperor, Section: Wang Jian.}}

• Ctesibius, Greek inventor and mathematician
• Eucleidas, king of Sparta (killed in the Battle of Sellasia)
• Ptolemy III Euergetes (the Benefactor), king of Egypt
• Viridomarus, military leader of the Gaesatae (Gaul)
• Xi of Yan, king of the Yan State (Warring States Period)

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