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Xerxes I of Persia is encouraged by his cousin and brother-in-law, Mardonius, supported by a strong party of exiled Greeks, to take revenge for the defeat that Darius I suffered at the hands of the Greeks at Marathon in 490 BC. In response, Xerxes prepares for a major expedition to crush the Greeks. To avoid a repeat of the significant losses to the Persian fleet that occurred in 492 BC, Xerxes has a canal cut through the promontory of Mount Athos.
Greece
Athenian archonThemistocles realises that the Greeks need to be able to beat the Persians at sea. To carry out this strategy, however, Athens needs far more warships (ie the newly developed, specialised triremes) than it then the 70 it had. Themistocles was initially opposed by other Athenian leaders. However, when the state-owned silver mines at Laurium become the site of a rich strike, Themistocles persuades the assembly, instead of "declaring a dividend," to devote the whole surplus to increasing the navy to a proposed 200 ships.
Sicily
Gelon, the tyrant of Syracuse conquers the nearby Sicilian cities of Euboea and Megara Hyblaea, selling their common people into slavery and bringing their oligarchs to Syracuse.