Meanwhile, the Sicilian Greeks, led by Gelo of Syracuse, successfully resisted and overthrew the aggression of Carthage, the issue being decided at the battle of Himera. The part played by Athens under the guidance of Themistocles in the repulse of Persia gave her a new position among the Greek states and an indisputable naval leadership. As the maritime head of Hellas she was chief of the naval Delian League, now formed ostensibly to carry on the war against Persia. But the leaguers, who first contributed a quota of ships, soon began to substitute money to provide ships, which in effect swelled the Athenian navy, and turned the contributors into tributaries. Thus, almost automatically, the Delian League converted itself into an Athenian empire. In Athens itself an unparalleled personal ascendancy was acquired by Pericles, who made the form of government and administration more democratic than before. But this growing supremacy of Athens aroused the jealous alarm of other Greek states. Sparta saw her own titular hegemony threatened; the subject cities grew restive under the Athenian yoke. Sparta came forward professedly as champion of the liberties of Hellas; Athens, guided by Pericles, refused to submit to Spartan dictation, and accepted the challenge which plunged Greece into the Peloponnesian war. The Athenians concentrated on the expansion of their naval armaments, left the open country undefended and gathered within the city walls, and landed forces at will on the Peloponnese. Platsea, almost their sole ally on land, held out valiantly for some time, but was forced to surrender; and Athens herself suffered frightfully from a visitation of the plague. After the death of Pericles, Cleon became the most prominent leader of the aggressive and democratic party, Nicias, of the anti-democratic peace party. Over most of Greece in each state the oligarchic faction favoured the Peloponnesian league, the democratic, Athens. The general Demosthenes at Pylos effected the surrender of a Lacedæmonian force, which temporarily shattered Sparta's military prestige, a blow in some degree counteracted by the brilliant operations of Brasidas in the north, where, however, both he and Cleon were killed.