The Peleponnesian War lasted 27 years. In 431 B.C.E Athens and Sparta found themselves at war with one another. Both sides believed the war would come to an end quickly, and both were wrong.
Athens had a large Navy and Sparta had a large Army. . Before the war began, Athens realized that they could not defeat Sparta by land, and Sparta realized that it did not have fleet capable of defeating the Athens at sea.
Athens had a wall built around the city so Sparta attacked the countryside and became frustrated when Athens did not fight back.
429 B.C.E a typhus epidemic killed over 1/3 of the populations of Athens. This plague also killed Pericles. Athens was weakened by this epidemic and the first of the Peloponnesian Wars ends as a stalemate.
IN 413 B.C.E a civil war began on the island of Sicily. Athens attempted to help destroy Syracuse, which was an ally of Sparta. Athens lost 200 ships and 50,000 men. Some of the allies that once sided with Athens, now were siding with Sparta. Sparta defeated Athens.
In 359 B.C.E Philip of Macedon rises to power. Macedon has cultural connections due to the Greeks, but Macedon were move primitive people from the North.
The Peloponnesian Wars weakened the city states which allowed Philip of Macedon to conquer and unite them.
In 411 B.C.E the city's government was replaced by an oligarchy which was chosen by a small assembly of citizens, democracy was reinstated 409 B.C.E.
Athens surrendered the Sparta, and Sparta became the ruling state of Greece. If the Greeks had become united, rather than engaged in war, they may have been able to withstand the Macedonians and others.