I certainly can not condense an explanation to three sentences, and it seems the custom here to keep post very short.  Donald Kagan's book "Pericles of Athens and the Birth of Democracy", uses a full chapter to explain the Parthenon, and that would be just the beginning of the explanation of change.  The change occurs over a long time before the Parthenon was built and following the building of the Parthenon.  Perhaps it is easier for you to explain to me, how Athens became a naval force that could defeat Persia and the political ramifications of meeting this urgent military need. 

Athens had help in defeating Persia, actually a lot of help. 🙂  Athens came to dominate a confederation of smaller surrounding city-states and subsequently began extracting tribute from them which lined their coffers.  Athens was as militaristic and opportunistic as Sparta in their drive for Peloponnesian hegemony.  Athens was also home of tyrants such as Pisistratus and Cylon who were no different than Sparta's Lycurgus or Leonidas.  Yes Athens experimented with constitutional democracy (see Aristotle and Xenophon), but that democracy was limited and very exclusive.

Perhaps it is easier for you to explain to me, how Athens became a naval force that could defeat Persia and the political ramifications of meeting this urgent military need. 

Athens was traditionally a naval power along with many of the other Greek city-states.  The explanation for why is simple, geography, it was much easier to travel around the rugged Greek peninsula by sea than walk.  there were only a few Greek city-states without naval traditions and Sparta was the largest of these.  The persians by contrast, were a land power.  There prestige rested on the strength and size of their army, they did not need a navy.  Indeed, a powerful navy would have been next to worthless for the Persians.  In naval power, Athens was just preeminent, they did not have a monopoly on naval ships, just the most ans the most experienced and best crews.  Part of the reason their crews were cinsidered so good is that their rowers were free men and not slaves.