The Peleponessian War (431-404 BCE) completely transformed Greek politics, economics, and culture. The war pitted the democratic Athens and its similarly minded allies with authoritarian Sparta and its allies, which saw the entirety of ancient Greece divided along geographic and philosophical lines in a contested series of cil wars. Sparta's eventual victory in the conflict made Athens destitute and greatly weakened their allies as well, restructuring the power based of the entire Greek lands.

Pericles is remebered as a champion of Athenian democracy in the fifth Century BCE. He led the Athenian democracy throughout much of its conflict with Sparta.

The Peloponnesian Wars were fought between Athens and Sparta in the fifth century BCE. The Greek city states had grown powerful and wealthy after defeating the invading Persians in the Persian Wars. Athens began to emerge as a hegemonic power in the region and worried Sparta with its expansionist tendencies. The two city-states went to war. The war ended in victory for the Sparta, destruction of Athens, and the disunity of the Greek city-states.

In the years after the Peloponnesian War, as the influence of the Greek city-states waned, the power of Macedon (to the north of Greece) rose spectacularly. Phillip of Macedon expanded Macedonian power greatly, and then his son Alexander the Great incorporated Greece into Macedonian control and proceeded to conquer much of the known world, reaching as far east as India.

The Ancient Greeks are often credited by historians with developing the world’s first representative democracy - particularly in the city state of Athens, where philosophy and democracy most prominently flourished. The Ancient Greek city-state was called a polis. 

Spartan society, as you probably know, was famous for its extreme militarization and organization. The Spartan army was the most accomplished in Ancient Greece and was victorious in numerous conflicts. These conflicts led to an influx of captured prisoners who were enslaved and used to work the fields and provide for the agricultural growth of Spartan society. They were called Helots. 

Solon and Draco are two notable political reformers in Ancient Athenian society. Before Solon democracy in Athens was virtually non-existent (indeed Solon himself was essentially a tyrannical figure). Solon instituted reforms to provide for direct democracy for many members of Athenian society. He also forbade debt-slavery. Draco is famous for instituting a code of laws in Athenian society - these laws were considered harsh, but just. 

In response to the continuous threat of Persian invasion in the fifth century B.C.E. Athens formed the extensive Delian League with dozens of islands and city-states dotted around the Aegean Sea and Asia Minor. The original purpose of the Delian League was to provide for a shared navy to protect against Persian invasion, but as the threat of Persian invasion waned Athens began to use the Delian League for its own purposes. This led to an outbreak of war with Sparta (and Sparta's Peloponnesian League) in 431 B.C.E.