A thorough analysis of a conflict involves an examination of, not only the actors and actions, but also an understanding the underlying causes. Although the ancient historian Herodotus is the "authoritative" source for the Persian Wars, he still provides insight as to the origins of the conflict.
Persian forces had pretty much overwhelmed Greek communities throughout Hellas and on the coast of Asia Minor. The onslaught of the Great Persian King (whomever held that position at the time) seemed to be unstoppable. Oddly, it was a misinterpretation of the Delphic oracle that sparked the initial dispute between Croesus and Cyrus, and a correct interpretation of the oracle, by Themistocles (Histories, Book VII, 141), which led to an Aeginian/Athenian victory at Salamis and the end of the wars (Histories, Book VIII, 40 -112).
The Lydian King, Croesus (pictured above), ruled from about 560 to 547 BCE according to Herodotus. He originally sought advice from the oracle of Apollo at Delphi regarding the outcome of a battle, should he decide to attack the Persian King Cyrus (pictured left). The oracle declared that "if he attacked the Persians he would bring down a mighty empire" (Histories, Book I, 91). Assuming that it would be the mighty Persian empire that was brought down, Croesus felt confident and began to attack the Persians. But after a few victories in the area that separated Lydia from Persia, Croesus' luck took a turn for the worse. At the Lydian city of Sardis, Croesus was defeated by Cyrus. (Histories, Book I, 84)
It has been noted by "Sergei Andropov" that the Persian Wars were more than just a conquest by successive Persian kings, power hungry for world domination. In fact, the wars were a series of reciprocal conflicts between the Persians and various Greek nations. It would be inaccurate to group all of these nations together and describe them as "Greece," because the Athenian led "Delian League" and the "Peloponnesian League," had not even been established at the time. Most of the Hellenic city-states considered themselves independent, and took pride in their own culture, customs and traditions. Andropov complains that:
"People tend to forget that Xerxes' purpose in invading Greece was not to conquer it (that being more of a side benefit), but to punish the Athenians for torching Sardis and thereby royally pissing off his dad, [Darius]"
But, what Andropov seems to forget is that Sardis was not originally under Persian control.
Here is a link to a map of Greece and the Eastern Mediterranean, click "Area 7" to see the location of Sardis in Asia Minor.
The Ionian Revolt was not successful at that time, however, but the spirit of Freedom left a lasting impression on Darius' twisted soul. Herodotus explains that:
[N]ews was brought to Darius that Sardis had been taken and burnt by the Athenians and Ionians, and that the prime mover in the joint enterprise was Aristagoras of Miletus. The story goes that when Darius learnt of the disaster, he did not give a thought to the Ionians, knowing perfectly well that the punishment for their revolt would come; but he asked who the Athenians were, and then, on being told [no doubt of the Freedoms that they (the Athenians) enjoyed], called for his bow. He took it, set an arrow on the string, shot it up into the air and cried: 'Grant, O God, that I may punish the Athenians.' Then he commanded one of his servants to repeat to him the words, 'Master, remember the Athenians', three times, whenever he sat down to dinner. (Book V, 105)
This fanaticism is the mark of an unhealthy ideology, and although the Ionian revolt failed, the Athenians (with Freedom on their side) would wind up driving the barbarians back into the desert.
It was Xerxes' navy that the Athenians defeated, and it could be argued that he was simply attempting to take revenge for the burning of Sardis. However, was revenge the only reason the Persians invaded Attica?
Plutarch provides a clue in his Life of Aristides,
When Datis was sent out by king Darius, on the pretext of punishing the Athenians for the burning of Sardis, but really to subdue the whole of Greece, he landed all his forces at Marathon and proceeded to ravage the countryside. (Life of Aristides, 5.1)
This means that Darius was 'upset' about the burning of Sardis, but according to Plutarch, the Great King used the incident as a pretext for conquest.