Greece had recently emerged victorious from the Persian Wars (499-449 B.C.). Perceiving themselves as the leaders of Hellas and superior to their neighbors, the Athenians worked to gain influence over the other city-states and dipped into the common treasury to build up their own city. Athens dealt with the opposition to its power by occupying several city-states in the 470s and 460s, leaving Sparta and Corinth as the only obstacles to Athenian supremacy. After a period of tensions, the Peloponnesian War (431-404 B.C.) finally broke out. The war resulted in Athenian defeat and what essentially amounted to the downfall of Greece.
Following the initial battles of the war, Athenian statesman Pericles delivered his famous funeral oration, which was recorded by the historians Thucydides (460-395 B.C.). See the link below. The speech reflects the Athenians' view of themselves as an open, educated, democratic, law-abiding, free, self-sufficient, and culturally superior society. Pericles uses the Spartans or Lacedaemonians as an opposite example. He implies their lack of openness, refinement, and self-sufficiency. Pericles also acknowledged their masculinity but suggests that it comes at the price of erudition, unlike in Athens.
Historians tend to agree that at the start of the war, Sparta has the stronger army and Athens had the superior navy. While Pericles had secured Persian neutrality before the war, Sparta's ability to secure Persian naval support during the war swung things in Sparta's favor. Given the outcome of the war, Pericles likely overestimated Athenian superiority over the Spartans.
For more on Athens and Sparta at the start of the war, examine Thucydides' History of the Peloponnesian War. See the second link below. He tends to see Athenian democracy as a weakness rather than a strength.