He was a very skilled leader. Skilled militarily, diplomatically and economically(more or less). He was also very skilled in reading circumstances and exploiting opportunities as they arose.

The Macedonian army, famed for conquering the known world, was built by Philip. Largely based on the lessons he learned from the innovative tacticians in Thebes, he pieced together a force that, initially, allowed him to stand his own ground against enemies from all sides(Macedon had been until then at the mercy of its neighbors and very near collapse); and eventually, through trial and error and adaptation, he was able to defeat, ally with, and gradually dominate the surrounding nations.

His control of the gold mines in the north of Greece(which were vast at the time) also put him in an enviable position of financial power, which he used to full effect on not only his army, but on bribes and gifts to other states and political factions, gaining favorable support in the Greek cities where he needed it. This would eventually play the biggest part in his success, as it split the Greek population into those who favored him, and those who opposed him. The Ancient Greeks were at their most effective when they were unified in opinion and will, the splitting of their opinions meant they could not effectively resist him, as they might have been able to otherwise. This gave him all the foothold he needed to hold Greece after he defeated their armies.

By the end of his reign, he was in charge of one of the strongest militaries in the world, with a vast array of allies and support troops, and had the support of one of the most economically powerful regions in the world at his call. His next goal was the conquest of Persia, a campaign which he had been planning for a while, until he was assassinated. This is the situation Alexander came into.