King Philip made the military a way of life for many Macedonian men. He transformed his army from a body that was little more than a weak, undisciplined amateur force into a tough, disciplined war machine. Philip's military innovations created the fighting power that Alexander inherited and perfected, making it an unsurpassed war machine Philip also helped Alexander understand the value of his corps of engineers in the development of new methods of siege warfare. Alexander revolutionized such techniques to great success. During Philip's expedition against Byzantium in 340 bc, when Alexander was sixteen years old, he left his son to serve as regent of Macedonia. Philip's main conquest was that of the Greek mainland. His vision was to create a "Hellenic community", a defensive alliance among the quarrelling Greek city-states and Macedonia that would be strong enough to overpower the Great King Darius of Asia Minor and liberate the Greeks there.