Leonidas handpicked 300 men to take with him to the Thermopylae, he was sent “in order that the sight of them might encourage the other confederates to fight and prevent them from going over to the enemy”. Leonidas as a young boy had been trained mentally and physically to fight in preparation to become a hoplite warrior, as a military leader handpicking his 300 men would have been a tactic used to find the strongest Spartan men who he trusted. The 300 Spartan men would have been equipped with a round shield, spear and iron short sword. In battle they used a formation called the phalanx in which rows if hoplites stood directly next to each so shields overlapped with one another. When the Persians attacked from the side and back of the phalanx, the Spartan formation would have been vulnerable which proved to be Leonidas weakness against the invading army. Leonidas although had a strong 300 Spartans he did not have enough tactics and men to stop a strong Persian army in battle, which limited his contribution to the Persian war.
Leonidas established his army at Thermopylae; this was a tactic which would funnel the Persian army towards his own force through a narrow pass. For a few days this tactic withstood the Persian army, Leonidas plan worked well. But Ephialates a local Greek is blamed by Herodotus as showing Xerxes a way over the mountains which allowed the Persians to surround Leonidas and his army. Leonidas sent all his allies except the 300 men and some