Why were Persia and Greece frequently at war? Why did the Persians eventually fail to defeat the Greeks?

Persia had a huge empire and had every intention of adding Greece to it. The Persian king Darius first attacked Greece in 490 BC, but was defeated at the Battle of Marathon by a mainly Athenian force. This humiliation led to the attempt to conquer Greece in 480-479 BC. The invasion was led by Xerxes, Darius's son.