In 480 BC, an enormous Persian army under the chief command of Emperor Xerxes (son of Darius the Great) campaigned against Thessaly in central Greece. Mainly they fought against the southern mostly democratic and independent city/states. The army numbered more than 300,000 men.
The Hellenes initially decided to defend themselves in Tempe valley (next to Mount Olympus) by sending about 10,000 fighters. Yet, a couple of months later they concluded that it was better to stand at the Thermopylae straits (about 150 km (93 miles) to the north of Athens), where, however, only a total of 7,000 hoplites could gather.
Artistic representation of the Battle of Thermopylae. (Internet Archive Book Images/Commons)
War During the Olympics
Like in Marathon 10 years earlier when the Spartans had their religious festival of Karnea dedicated to Apollo, at the end of summer in 480 BC people from all over the Hellenic lands (including those in Africa and Sicily) participated to the Olympic Games. The Games were dedicated to Zeus and war was forbidden when they were held.