Answer
Phalanx formation was the primary tactic employed by Leonidas I at Thermopylae. This formation involved tightly packed rows of heavily armed infantrymen, with overlapping shields and long spears. The phalanx created a formidable defensive wall, allowing the Spartans to withstand the overwhelming numbers of the Persian army. By holding the narrow pass at Thermopylae, Leonidas and his men effectively blocked the Persian advance, despite their eventual defeat.