Athens and Sparta always hated each other because they both have always wanted to rule all of Greece 

Sparta thought their military would easily crush any enemy in their way, but Athens thought that they could dominate Sparta with their navy

However, Sparta and Athens failed to notice that Greece was slowly falling apart due to their constant fighting

Greece had become weak and vulnerable (easily attacked or harmed)

Since Greece was in a weak and vulnerable state, Sparta and Athens made a truce that was supposed to last 50 years but it only lasted 2 years 

They were drawn back to war over the rich lands of Sicily (each wanted to control it) 

The Spartans prompted Athens to intrude on them

Athens laid siege of Syracuse, Sicily’s strongest and richest city

However, the Athenians made many mistakes and the Spartans attacked them on sea and on land

The Spartans sank all 200 of their ships and killed or enslaved 40,000 soldiers

Sparta took control of all of the areas outside of Athens and cut off their agricultural and economic resources 

Athens was being destroyed and they really could not fight back because Sparta had also destroyed their navy

Not only that, Sparta had gotten stronger since they allied themselves with one of their old enemies, Persia

Sparta’s army steadily advanced on Athens and left them with 2 choices: surrender or starve

The Peloponnesian war was finally over

Members of the Peloponnesian League wanted to to see Athens completely destroyed (the surrounding Greek city-states resented them for their greed)

Sparta did not listen to the other Greek city-states and did not destroy Athens (they felt that Athens was humiliated enough already)

Sparta replaced Athens' democratic government with an oligarchy (government rule by a small group of powerful elites)

Ruthless tyrants ruled now

Greece was left weak due to the many battles, loss of men, and resources

Greek city-states began warring with each other

Greece was left perfectly open for attack in this weakened state