In the 440s Pericles won his way to the top in the contest for leadership in democratic Athens. It was a time of almost nonstop conflict in foreign affairs and domestic politics. The Spartans followed up their rejection of Pericles' proposal for Greek unity by sending a military expedition to Delphi to secure their own primacy at Apollo's shrine. The Athenians countered this move after the Spartans returned home by sending Pericles to command an expedition to the god's sanctuary to reverse the arrangements that their enemies had made. He had the name of the Athenians inscribed on the statue of a bronze wolf in the sacred precinct to show that petitioners from Athens received the privilege of going to the head of the line of people waiting to consult the oracle, displacing the Spartans from that treasured advantage. The five-year peace that Athens and Sparta had agreed to in 451 was obviously fraying.

As usual, the chronology of events is confused in the ancient sources, but it is clear that Pericles was now playing a major role as a military commander in campaigns outside Athenian territory. Still, Athens had no second Cimon in the sense of a general whom the people regarded as far and away their best, and Tolmides' military reputation seems to have been as prominent as Pericles' in the early years of the decade. Pericles did, however, win great acclaim for the expedition he led in about 447 to the Thracian Chersonese. Ancestors of Cimon had long before settled Athenians on this peninsula, whose cities controlled the strait through which crucial imports of grain and timber were transported south from the shores of the Black Sea. The Thracians had never been happy about this encroachment, and they constantly raided the Greeks' settlements.

Pericles reestablished the Athenian presence in this strategically important region by recruiting and transporting a large group of new settlers to occupy the land and building a defense wall four miles long to reach from shore to shore across the neck of the peninsula. It seems possible that Pericles was aided in this innovative plan by recalling information about the region that he had long before learned from his father, who had been the Athenian commander in a victorious campaign in the Chersonese at the end of the Persian Wars.