Aegean Sea for its fleet.
Dardanelles Strait for its army.
The Persians and their Greek allies were united to prosecute the war. Many Greek city-states were united against the Persian and allied forces.
It removed the Persian attempt to incorporate the Greek city-states into the Persian Empire.
The conquerors of the Greek and Persian were Greeks, Persians, various Asiatics, Romans, Muslims, Turks, Mongols and others.
The Persian king, and first Sparta then Athens on the Greek side.
Alexander the Great conquered the Persian Empire and converted it into an empire of his own, not to destroy the Persians but to convert them to Greek culture.
The Persian Empire versus varying combinations of Greek city-states.
The Ionian Greek city states of Asia Minor who had revolted against Persian rule.
Some sided with the Persians, others combined to repel the Persian invasion.
Their infantry warriors wore metal armour, which the Persians did not.
There would have been ongoing Greek revolutions.
They were leaders the Persian Empire. Darius was the father of Xerxes. Darius led the Persians in the first Persian war, while Xerxes led the Persians in the second Persian war.
Most of the Persian defeats in the 50 years of warfare between the Greek city-states and Persia were either sea or sea-land battles, not land battles involving just the army. The single land battle of Plataia in 469 BCE was won by the Greek forces sticking defeating the inferior unsupported Persian infantry.