The Persian army was one of the largest and most sophisticated invading armies ever formed in the ancient world

One of the most interesting facts about the Battle of Thermopylae is that the Persian army was one of the largest and most sophisticated invading armies ever formed in the ancient world. The number of troops assembled by Xerxes for his second invasion of Greece has been the topic of unending debate, particularly between ancient sources, which record very huge numbers, and modern academics, who estimate considerably smaller proportions. According to Herodotus, there was 2.6 million military personnel in total, along with an equal number of support people. The poet Simonides, who lived at the time, speaks about four million; Ctesias gives 800,000 as the overall number of Xerxes' army.

Modern researchers prefer to dismiss Herodotus' and other ancient authors' statistics as unrealistic, the consequence of victors' miscalculations or exaggerations. These estimations are typically derived from research into the Persians' logistical capabilities at the time, the sustainability of their different bases of operations, and the general manpower restrictions influencing them. Whatever the true numbers were, it is evident that Xerxes was eager to assure the expedition's success by amassing an overwhelming numerical advantage on land and water. As a result, the number of Persian troops present at Thermopylae is as unknown as the entire invading force. It is unknown, for example, whether the entire Persian army marched to Thermopylae or whether Xerxes left garrisons in Macedon and Thessaly.