By place

Greece

• May — King Xerxes I of Persia marches from Sardis and onto Thrace and Macedonia.
• The Greek congress decides to send a force of 10,000 Greeks, including hoplites and cavalry, to the Vale of Tempe, through which they believe the Persian army will pass. The force includes Lacedaemonians led by Euanetos and Athenians under Themistocles. Warned by Alexander I of Macedon that the vale can be bypassed elsewhere and that the army of Xerxes is overwhelming, the Greeks decide not to try to hold there and vacate the vale.
• August 11 — The Battle of Thermopylae ends in victory for the Persians under Xerxes. His army engulfs a force of 300 Spartans and 700 Thespiae under the Spartan King, Leonidas I. The Greeks under Leonidas resist the advance through Thermopylae of Xerxes' vast army. For two days Leonidas and his troops withstand the Persian attacks; he then orders most of his troops to retreat, and he and his 300-member royal guard fight to the last man.
• A member of the Agiad royal family, and the son of King Cleombrotus and nephew of King Leonidas, Pausanias becomes regent for Leonidas' son, Pleistarchus, after Leonidas I is killed at Thermopylae.
• Phocis and the coasts of Euboea are devastated by the Persians. Thebes and most of Boeotia join Xerxes.
• King Alexander I of Macedon is obliged to accompany Xerxes in a campaign through Greece, though he secretly aids the Greek allies. With Xerxes' apparent acquiescence, Alexander seizes the Greek colony of Pydna and advances his frontiers eastward to the Strymon, taking in Crestonia and Bisaltia, along with the rich silver deposits of Mount Dysorus.
• The Athenian soldier and statesman, Aristides, as well as the former Athenian archon Xanthippus, return from banishment in Aegina to serve under Themistocles against the Persians.
• August — The Persians achieve a naval victory over the Greeks in an engagement fought near Artemisium, a promontory on the north coast of Euboea. The Greek fleet holds its own against the Persians in three days of fighting but withdraws southward when news comes of the defeat at Thermopylae.
• Breaking through the pass at Thermopylae from Macedonia into Greece, the Persians occupy Attica.
• September 21 — The Persians sack Athens, whose citizens flee to Salamis and the Peloponnesus.
• September 28 — The Battle of Salamis brings victory to the Greeks, whose Athenian general Themistocles lures the Persians into the Bay of Salamis, between the Athenian port-city of Piraeus and the island of Salamis. The Greek triremes then attack furiously, ramming or sinking many Persian vessels and boarding others. The Greeks sink about 200 Persian vessels while losing only about 40 of their own. The rest of the Persian fleet is scattered, and as a result Xerxes has to postpone his planned land offensives for a year, a delay that gives the Greek city-states time to unite against him.
• An eclipse of the sun discourages the Greek army from following up the victory of Salamis. Xerxes returns to Persia leaving behind an army under Mardonius, which winters in Thessaly.

Roman Republic