Why were the greek city-states open to attack by philip after the peloponnesian war?

The correct answer is that Greek city-states were too weak and disorganized to unite after the Peloponnesian War

The war took too much of a toll on them which enabled Philip to start attacking them and uniting them into a single empire. He was unsuccessful however because of his premature death but the process ended with his son Alexander, also known as Alexander the Great, who managed to unite all of Greece into a single empire.