After the first Persian invasion and subsequent loss to the Athenians at the Battle of Marathon, Persian King Xerxes I amassed a huge army and navy for a second invasion along the western coast of the Aegean Sea. His plan was to overwhelm and squash the Allied Greeks once and for all. The Greek Allies, anticipating this move, had been building a fleet of triremes (battle boats used primarily to ram other boats and transport fighters) in 482 BC under the guidance of the Athenian politician Themistocles. Athenians realized that it would take an allied Greek force to repel the Persians. Uncharacteristically, as most of the city-states were at odds with each other, the city-states came together to create an allied force under the direction of a make-shift "congress."
The allied forces initially sent ten thousand hoplites (citizen-soldiers, mostly armed with shield and spear) to stop Xerxes at the Vale of Tempe. Alexander I of Macedonia, a Persian vassal, alerted the Greek troops that the Persian army was massive and taking another route. The hoplites withdrew and Themistocles came up with another plan. In order for the Persians to access Boeotia, Attica, and Peloponnesus, they would either have to go by land through a narrow pass in Thermopylae (the ‘Hot Gates’) or go by sea through Artemisia. The Persian force attacked both places simultaneously.
At Thermopylae, a few thousand hoplites led by King Leonidas I of Sparta held the Persian army for three days with less than a tenth of the casualties. Once Thermopylae fell, holding Artemisia was no longer an issue. Most of the Allied Greek forces in Artemisia then retreated to the Saronic Gulf (eastern side of the Isthmus of Corinth in the Aegean Sea). The Persian troops now had their access routes and used them to slash and burn their way to the evacuated Athens. The Allies withdrew to blockade the Isthmus of Corinth and Themistocles had another idea, to lure the Persians into the Straits of Salamis. Similar to the strategy used at Thermopylae, the straits would force the Persian fleet into a tight fighting area.
Roughly 366 triremes from 21 Greek city-states blocked off the Straits of Salamis from 600 to 800 Persian vessels. Persian forces were nearly half of their original number at this point as they had suffered heavy losses in storms off the coasts of Magnesia and Euboea and from the Battle of Artemisium. Xerxes knew he had to crush the Greeks in a decisive victory and quickly. He had not anticipated the Greek resistance to be very strong and had not planned on the campaign extending over a season. Despite the desire to end the invasion quickly, Xerxes was counseled that he did not have to engage the entire navy in the straits as the Persian fleet was large enough to send half in to deal with the Greek fleet in Salamis and send the other half around the islands to invade Peloponnesus. With tactical advantage and larger numbers, Xerxes sent his whole naval invasion force into the straits confident of victory.
The Greek Allies separated into at least two groups as the straits were too narrow to allow the entire fleet to line up. As the Persian fleet entered into the straits and fought against the Greek fleet, chaos erupted. When the first row of Persian vessels engaged the Greeks, some were damaged and needed to retreat behind Persian lines. As this first row retreated, they became fouled in the second and third row of Persian ships. When the second row of Persian ships retreated, they became tangled in the third wave, creating a ripple effect of disorganization. The left flank of the Persian fleet lost their commander (Xerxes’ brother Ariabignes) early in the fighting, leaving the fleet without direction. Much of the Persian left flank was pushed back and run aground. A contingency of Greek vessels ran a wedge up the middle of the Persian fleet, separating it into two halves. The Persians were outmaneuvered and out fought as Greek vessels carried hoplites unlike the lightly armored Persian sailors. On their retreat to Phalerum, the Persian fleet sustained more losses as it was ambushed by Aeginetans.
Scholars estimate that over 200 Persian ships were lost in the Straits of Salamis. Xerxes didn’t give up though. He continued to try to build pontoon bridges across the straits but the area was too heavily patrolled. Finally conceding, Xerxes retreated further north and left General Mardonius in charge of the invasion over the winter. In the summer of 479 BC, the Greek Allies assembled a large army to meet the Persians in Boeotia, leading to the Battles of Mycale and Plataea and the eventual expulsion of the Persians from Greek Allied territory. The Battle of Salamis was a turning point in the second Persian invasion proving that the Allied Greeks could and would defend themselves successfully from outside attack. The loss at Salamis coupled with those at Mycale and Plataea marked the eventual lessening of Persian power throughout the Aegean region.