Sparta was pressured to attack Athens. This conflict was the Peloponnesian War because Sparta was located in the Peloponnesus.
In the war's first winter, Pericles gave a speech called a Funeral Oration. He was honoring soldiers that died in battle. He emphasized that the democratic way of life is worth protecting.
Pericles's Funeral Oration
In one of the battles, Sparta surrounded Athens. Pericles chose to keep the Athenian army within the walls of the city. After 2 years of safety, a disease broke out and killed more than a third of the people, including Pericles. The next 25 years were pretty equal between Athenian and Spartan victories.
Sparta made a deal with the Persians and Persia gave them money to build a navy. Eventually Athens's democracy was overthrown. The Spartan navy destroyed the Athens navy.
Athens :( Part 2
Sparta placed a blockade around Athens and prevented food and supplies from entering. Starving, the Athenians finally surrendered. The Spartans had won the war and the Athenian empire collapsed.
Athens :( Part 3
After the war, governments were divided and weak. Many people died during battle or from disease. Sparta was weak and was taken over by Thebes and eventually by the kingdom of Macedonia.