Passage: By contrast, Philip treated Athens very leniently; although the Second Athenian League was dissolved, the Athenians were allowed to keep their colony on Samos, and their prisoners were freed without ransom. Philip's motives are not entirely clear, but one likely explanation is that he hoped to use the Athenian navy in his campaign against Persia, since Macedon did not possess a substantial fleet; he therefore needed to remain on good terms with the Athenians. Philip also made peace with the other combatants; Corinth and Chalcis, which controlled important strategic locations both received Macedonian garrisons. He then turned to deal with Sparta, which had not taken part in the conflict, but was likely to take advantage of the weakened state of the other Greek cities to try to attack its neighbours in the Peloponnese. The Spartans refused Philip's invitation to engage in discussions, so Philip ravaged Lacedaemonia, but did not attack Sparta itself.Philip seems to have moved around in the months after the battle, making peace with the states that opposed him, dealing with the Spartans, and installing garrisons; his movements also probably served as a demonstration of force to the other cities, that they should not try to oppose him. In mid 337 BC, he seems to have camped near Corinth, and began the work to establish a league of the city-states, which would guarantee peace in Greece, and provide Philip with military assistance against Persia. The result, the League of Corinth, was formed in the latter half of 337 BC at a congress organised by Philip. All states signed up to the league, with the exception of Sparta. The principal terms of the concord were that all members became allied to each other, and to Macedon, and that all members were guaranteed freedom from attack, freedom of navigation, and freedom from interference in internal affairs. Philip, and the Macedonian garrisons installed, would act as the 'keepers of the peace'. At Philip's behest, the synod of the league then declared war on Persia, and...

Question: What did Philip work to establish?
league of the city-states