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[[File: 512px-Filip II Macedonia.jpg|thumbnail|300px|left|A bust of Phillip II]]
As a young man, Phillip was sent as a hostage to Thebes. At the time, it had the most powerful army in Greece and the Theban army had defeated the once invincible Spartans at the Battle of Leuctra in 373 BCE. Phillip was a shrewd young man and he spent his time in Thebes studying its army and especially the tactics of the renowned Theban commanders Epaminondas and Pelopidas. He also saw how a phalanx could be utilized to drive an enemy from the battlefield. In particular, he studied the famous ‘Theban wedge’ and this was a mass attack over open ground by a phalanx at pace. Phillip was also deeply impressed by the Theban elite force the ‘Sacred Band’ and he was later to model his own elite forces on this unit. Using these experiences, he completely reorganized Macedonia’s army. When he became regent of Macedonia he increased the size of the army and indeed doubled its size. He also greatly expanded the cavalry and trained the army in the latest Theban tactics. He ensured that the Macedonians were well drilled and disciplined. He also made sure that the army was regularly paid and received other benefits and he transformed the army from one, that was a militia of citizen-soldiers to one composed of professional soldiers. He provided his army with uniforms and every soldier gave him an oath of allegiance. Phillip II also established a corps of engineers and they were soon to prove adept at siege warfare.
Phillip learned much from the Thebes but he also introduced his own tactics. He adapted the phalanx and he made it more flexible. He appointed a commander to every phalanx and established a system for communications. The Macedonian King also adapted the equipment of his army. He abandoned the traditional spear and introduced a very long spear, up to twenty feet in length. It meant that the Macedonian phalanx could reach the enemy before they were within reach of their spears. He also equipped every soldier with a short sword, that was ideal for close combat. The army that Phillip II developed was to help him establish an empire. It was this army that allowed him to turn Macedonia from a second-rate power into the major Greek power. It was this army that allowed Alexander to conquer most of the known world<ref>Lewis. p 101</ref>.