For much of Ancient Greek history, the political arrangement of Greece was that of independent city-states with their own autonomy. This tradition began with the Mycenaean civilization and existed because of how the mountains created regionalism within Greece. The two most powerful city-states, Athens and Sparta, were plunged into a thirty-year conflict known as the Peloponnesian War (431-404 BC.) The Peloponnesian War was won by Sparta but had the effect of weakening most of the city-states of Greece.
The Peloponnesian War weakened the city-states to the point that they were vulnerable to attack. The Macedonians to the north were quietly building a strong military state. Under Philip II, the Macedonians conquered many of the city-states and were unified under one ruler in 337 BC under the League of Corinth. Philip was assassinated in less than a year and his son, Alexander, ascended to the throne.