Alright the Greeks are absolutely bad ass.  Developing on a really rocky, and resource lacking, peninsula that jets out into the Mediterranean, it was obvious from the start that in order to ensure survival trade would be a necessity.  However, to the east the Greeks stumbled upon the absolutely mammoth empire, one larger then the world had ever seen.  The Persian Empire, being the size it was, was obviously politically well structured, at least in comparison to the Greeks who were just a bunch of city-states who would chill with each other as well as quarrel with themselves from time to time.  Due to Persia's large grasp on much of the known world, they not only had conquered land but ideas, many of which involved technological advancements that easily surpassed the seemingly less might Greek peoples.  These technological advancements, some of which involved transportation, allowed the Persians to manage their massive empire, however the political system that they developed worked seamlessly.  Essentially the empire was split up into 20 provinces and each province was managed by someone who was close to the Kings (Darius 1) family.  Each court was set up like the main Persian court, which resulted in a universal acceptance of the Persian government.  The other astounding thing that the Persians were able to do was manipulate religion to fit their political agenda.  Pretty much everyone in Persia loves Zoroastrianism, so obviously the government (I think it was Darius that did the most with this), used the underlying ideas that the religion presents to justify their desire to conquer everything.  Zoroastrianism essentially tells of a once perfect world that is in a long fight with evil, and humans need to help restore the world to unity.  So Darius pretty much says "Lets just conquer the world and make everything peaceful".  So when the Persians come up to the Greeks they just see them as another people that will soon be annexed into the Empire.