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 many deaths, including that of Pericles. At the same time the Athenian fleet landed troops in the Peloponnesus, winning battles at Naupactus (429) and Pylos (425). However, these tactics could bring neither side a decisive victory. After several years of inconclusive campaigning, the moderate Athenian leader Nicias concluded the Peace of Nicias (421).