After the Persian Wars end, Athens and Sparta become the two major powers in Greece. When the Persian Was end, Sparta angers the other Greek poleis. Athens creates the Delian League to be prepared for possible Persian attacks.
The Spartans are suspicious of What Athens will do with its increased power. The Spartans react. They force many poleis to join a league run by them. The Spartan Peloponnese League is little but the Spartans using their military muscle. Sparta's allies are forced to fund Sparta's foreign policy and defense.
A thirty-year peace is arranged between Sparta and Athens. It is not long before the Spartans say the Athenians have broken the peace. More important is the fact Sparta feared Athens more every year. The Spartans decide to attack now before Athens might gets stronger.
The Spartans pulls its "allies" together and marches into Attica. The Athenians put their money into their navy. They just withdraw within their walls. Athens has huge, strong walls.
This war involves a new type of war. The Spartans can have a year round army because they have 200,000 Messenian helots doing all the work. Spartans are totally professional soldiers.
Athens' fleet grows larger and larger. Serving in the military in Athens soon comes to be seen as serving in the Athenian navy. This large navy is not allowed to lay around. Athens uses its fleet to force Delian League members to follow Athens' lead.
Sparta can bottle up the Athenians within Athens. It cannot take the city. Athens can feed its people on imported food brought by its navy. It can attack using its navy. It cannot defeat the huge Spartan Army. After ten years of this stand off, a peace is arranged.
The proud Athenians ruin the peace. They attack in the Peloponnese and in the Aegean. Sparta returns to war once again attacking Athens.
The Athenians decide to attack the colony of Syracuse in southern Italy. This is a disaster. The commander of the expedition is ordered arrested on way to Italy. He escapes arrest. He runs off the Sparta, and he spends years advising them on how to finish off Athens. His first suggestion: build a huge navy. Sparta goes to the Persians got the cash. The Persians are glad to pay for anything that will destroy Athens and its navy.
This is the end for Athens. The polis is hit by a terrible disease, a plague. One third of the people in Athens die. This weaken their ability to fight. Then the Athenian navy is destroyed by the new Spartan navy.
Athens has lost. It surrenders. Spartan soldiers enter Athens. The Spartans have the walls of Athens torn down. They have have what remains of the Athenian navy destroyed. They stop all trade between Athens and the world. They take away all of Athens' colonies. The Spartans are also quick to abolish democracy. They demand Athens be ruled by an oligarchy of tyrants.
Sparta ends the war in control of all of Greece. What they do with all this power determines the Spartans fate.