Why was Philip able to attack the Greek city-states?

Answer 1

Philip II of the ancient Greek kingdom of Macedonia, never conquered Greece. He brought the Greek states together, some by diplomacy and negotiations, others by force and united Greece under Macedonian Hegemony.
As the eminent historian J. B. Bury writes:

• As the hegemony or first place among Greek states had passed successively from Athens to Sparta, and to Thebes, so now it passed to Macedon. The statement that Greek liberty perished on the plain of Chaeronea is as true or as false as that it perished on the field of Leuctra or the strand of the Goat's River. Whenever a Greek state became supreme, that supremacy entailed the depression of some states and the dependency or subjection of others. Athens was reduced to a secondary place by Macedon, and Thebes fared still worse; but we must not forget what Sparta, in the day of her triumph, did to Athens, or the more evil things which Thebes proposed.

Answer 2

Philip of Macedon was able to attack the Greek city-states for several reasons. First, he implemented a highly organized and disciplined military, which gave him an advantage over the less coordinated Greek forces. Second, he used diplomacy and strategic alliances to weaken his enemies. Lastly, he took advantage of political instability and rivalries among the city-states to exploit their internal divisions.