In 481 BC massive preparations, shipbuilding and preparing of supplies and magazines were in train. These preparations were quite open and the Persian aim was obviously to subjugation of Greece.
By the Winter of 481/480 BC Xerxes preparations were complete and his forces gathered at Sardis before crossed the Hellespont.
Xerxes personally led the Persian invasion of Greece with one of the largest ancient armies ever assembled. According to Herodotus there were 1.7 million strong man from Persia.
The Athenians sent a messenger to ask the Spartans to commit their army, an army which could be considered a special forces unit in modern terms. Normally the special forces make up only a small portion of the military. The Spartans, however, were all highly trained killing machines. The Spartan king Leonidas knew that if the Spartans did not show up then the Persians would easily conquer Athens, and eventually Sparta.
Victory over the allied Greek states at the famous Battle of Thermopylae allowed the Persians to torch an evacuated Athens and overrun most of Greece.
Heroic Spartan resistance at Thermopylae failed to prevent Xerxes from sacking Athens, but his navy was mauled at Salamis and in 479 Greek hoplites were victorious at Plataea.
Persian invasion of Greece in 480 BC
