The main belligerents in the Peloponnesian War, which altered the power dynamics in ancient Greece, were Athens and Sparta. This conflict (431-404 BC) stemmed from rivalry and competition for dominance among Greek city-states. Athens, with its powerful navy and trade network, represented democracy, while Sparta, known for its military strength and oligarchic government, symbolized a different social structure. The war ultimately led to the decline of Athenian power and the reshaping of the Greek world, paving the way for rising powers like Macedonia.