Over the winter, there was some tension among the Allies. In particular, the Athenians, who were not protected by the Isthmus, but whose fleet was the key to the security of the Peloponnesus, felt that they had been treated unfairly, and so they refused to join the Allied navy in the spring. Mardonius remained in Thessaly, knowing an attack on the Isthmus was pointless, while the Allies refused to send an army outside the Peloponessus. Mardonius moved to break the stalemate, by offering peace to the Athenians, using Alexander I of Macedon as an intermediate. The Athenians made sure that a Spartan delegation was on hand to hear the Athenians reject the Persians' offer. Athens was thus evacuated again, and the Persians marched south and re-took possession of it. Mardonius now repeated his offer of peace to the Athenian refugees on Salamis. Athens, with Megara and Plataea, sent emissaries to Sparta demanding assistance, and threatening to accept the Persian terms if they were not aided. In response, the Spartans summoned a large army from the Peloponnese cities and marched to meet the Persians.