Thinking Classical Greece, pre-Philip II. Economically it seems like Persia should have annihilated Greece, were their military defeats just luck for the Greeks?
I know the military history, trying to understand how it could have gone that way.
I've written some replies on this topic before:
And in reply to your specific question:
There are several different aspects to look at.
First of all, the Achaemenids had an excellent logistical system in place. The Persepolis fortification tablets give an in-depth insight into this organization. Taxes of food, goods and money were collected from the Empire and deposited in a number of central locations, such as the Achaemenid royal and satrapal capitals. From there they were distributed to others as payment for services, and these payments were records on clay tablets so scribes could keep track of expenditure:
http://www.livius.org/sources/about/persepolis-fortification-tablets/?
So the Persians had huge amount of stores, and had the records so one could see what was stored and where. For military campaigns, these supplies were probably drawn on as the army marched from store-point to store-point. It was relatively easy to do this on the same continent, but difficult to do say when separated by an ocean. The Persians in Greece were dependant on external supplies, but as the Greeks repeatedly achieved naval supremacy, this limited the amount of time a Persian army could operate. Likewise only so many forces could be transported and supplied by ships, and this would limited the forces that could be brought to bear. Don't forget that Greece was on the very edge of the Empire, away from the heartlands of Persia and Media, and large numbers of Achaemenid troops were tied up in garrisons, once more limiting the number of soldiers available. The force that was defeated at Marathon was tiny by Persian standards. They had 20,000 soldiers in the standing army, alone.
Secondly, from the Persian perspective they could claim to have achieve their goals during Xerxes rule. Think about it. Sparta and Athens had to be punished. A large Spartan force (not just 300 Spartan citizens, but hundred of helots and thousands of Greeks) was defeated and their king killed, and Athens was captured, looted and burned. In ideological terms Xerxes could point to these achievements as proof of his success. The Persian defeat at Plataia could be blamed on Mardonious, thereby Xerxes' reputation remained intact.
Sources
Ancient Persia: A Concise History of the Achaemenid Empire, 550-330 BCE, by Matt Waters.
Empire, Authority, and Autonomy in Achaemenid Anatolia, by Elspeth R. M. Dusinberre
The Histories, by Herodotus: http://onlinebooks.library.upenn.edu/webbin/gutbook/author?name=Herodotus
The Persian Empire, by Lindsay Allen
Follow up question: How accurate is fire of Persia by tom hollard at answering this question?