The Athenians had been unable to conquer Amphipolis, which commanded the gold mines of Mount Pangaion. So Philip reached an agreement with Athens to lease the city to them after its conquest, in exchange for Pydna (lost by Macedon in 363). However, after conquering Amphipolis, Philip kept both cities (357). As Athens had declared war against him, he allied Macedon with the Chalkidian League of Olynthus. He subsequently conquered Potidaea, this time keeping his word and ceding it to the League in 356.