How a battle came to be fought at Salamis
The reasons for the Persian War were not terribly complex. However, they were compelling. Xerxes, the great and mighty king of the Persian Empire, had recently come to power. He was part of a dynasty whose rule almost required military glory and continuous expansion in order to justify its existence. Even so, it was not as if the Greeks did not provoke the Persians. They had been fomenting trouble in Persia's western dominions known as Ionia (on the west coast of Turkey).
This was actually the second time the Persians had chosen to fight the Greeks. The first time they had been stymied at Marathon, a city just over twenty-six miles north of Athens. That invasion had been under the father of Xerxes, Darius. It had taken some twenty-years for the Persians to make another try. This time, however, the Persians would take no chances under-estimating the Greeks. Xerxes raised an army of over 100,000 men (an astronomical figure in the ancient world). He kept his army close to the coast and supplied by sea. To ensure he kept his sea lanes opened, he had a fleet of over 1200 triremes outfitted for this adventure.
The Persian army marched through Thrace, Macedonia and then hit a brick wall at a mountain pass called Thermopylae. In this famous battle some 300 Spartans under King Leonidas held off the entire might of the Persian army for three days. It took the traitorous action of Ephialtes, a local Greek, to guide part of the Persian army around the Spartans through mountainous terrain. Thus they surrounded the Spartans and completely anialated their force.
The stubborn resistance of Leonidas worried Xerxes. But he had even bigger worries in the form of the Greek Navy. This force, about 300 strong, made up mostly of Athenian triremes under the command of Themistocles, was causing trouble in the king's fleet. An action was fought at Artemisium where many were lost. Weather, too, took its toll on the exposed Persian fleet, destroying hundreds of the fragile craft.
But the Persian superiority in manpower and ships carried the Persians down to Athens where the citizens had already escaped en mass to Salamis, an island off the coast of Attica. The Persian army was still vastly superior, but the differences in the number of ships between the Greeks and Persians was becoming more balanced. The Persians burnt Athens and destroyed much of the surrounding countryside.
This was fine for the Persians except for one thing. They could not live off the land. To survive they needed to have supplies shipped from the home country. Both sides understood the crucial nature of supply. Both sides understood that this gave the Greeks their only real opportunity to win the war. They must use their fleet to destroy the Persian supply ships. To stop the Greeks, the Persian fleet had to destroy the Greek Navy. This set the stage for the battle of Salamis.