During the decade, Themistocles continued to advocate the expansion of Athenian naval power. The Athenians were certainly aware throughout this period that the Persian interest in Greece had not ended; Darius's son and successor, Xerxes I, had continued the preparations for the invasion of Greece. Themistocles seems to have realised that for the Greeks to survive the coming onslaught required a Greek navy that could hope to face up to the Persian navy, and he therefore attempted to persuade the Athenians to build such a fleet. Aristides, as champion of the "zeugites" (the upper, 'hoplite-class') vigorously opposed such a policy.