How did they feed 600,000 (or even 300,000) men while marching or sailing around the ancient world?
It is really hard to believe they had the logistical or food preservation skills to do that in 380 B.C.
Even if they confiscated 100% of the food that crossed the path of the army, it still does not seem possible to survive.
Take a look at the size of the Persian Empire, consider how large the Persian army must have been all the time (not just during a campaign of this kind), and note that the locals were bowing and scraping to sell them some of their own supplies to avoid having all of them commandeered. That "drank whole rivers dry" wasn't much of an exaggeration. It was a big force, and the logistical skills and civil engineering (bridges, roads, harbors) of the Persians was impressive, just as the Romans' was.
you are correct. Greece imported a lot of their food, so the Persian Army couldn’t live off the land, but would have had to have their ships bring in supplies.
the Battle of Salamis decimated their fleet and probably contributed to their ultimate withdrawal.