Sparta's initial strategy was to invade Attica, but the Athenians were able to retreat behind their walls. An outbreak of plague in the city during the siege caused heavy losses, including the death of Pericles. At the same time the Athenian fleet landed troops in the Peloponnese, winning battles at Naupactus (429 BC) and Pylos (425 BC). But these tactics were not able to provide either side with a decisive victory. After several years of inconclusive campaigning, the moderate Athenian leader Nicias concluded the Peace of Nicias (421 BC).