Despite the treaty, Athenian relations with Sparta declined again in the 430s, and in 431 BC the Peloponnesian War began. The first phase of the war saw a series of fruitless annual invasions of Attica by Sparta, while Athens successfully fought the Corinthian empire in northwest Greece and defended its own empire, despite a plague which killed the leading Athenian statesman Pericles. The war turned after Athenian victories led by Cleon at Pylos and Sphakteria, and Sparta sued for peace, but the Athenians rejected the proposal. The Athenian failure to regain control of Boeotia at Delium and Brasidas' successes in northern Greece in 424 improved Sparta's position after Sphakteria. After the deaths of Cleon and Brasidas, the strongest proponents of war on each side, a peace treaty was negoitiated in 421 by the Athenian general Nicias.