Philip was now busy with preparations for the vast projects which he contemplated, and which embraced an attack upon the Athenian colonies, as well as upon the Persian empire. For this purpose he had organized a considerable naval force as well as an army; and in the spring of 342 B.C. he set out on an expedition against Thrace. His progress soon appeared to menace the Chersonese and the Athenian possessions in that quarter; and at length the Athenian troops under Diopithes came into actual collision with the Macedonians. In the following year Philip began to attack the Greek cities north of the Hellespont. He first besieged and captured Selymbria on the Propontis, and then turned his arms against Perinthus and Byzantium. This roused the Athenians to more vigorous action. War was formally declared against Philip, and a fleet equipped for the immediate relief of Byzantium. Philip was forced to raise the siege not only of that town but of Perinthus also, and finally to evacuate the Chersonesus altogether. For these acceptable services the grateful Byzantians erected a colossal statue in honour of Athens. After this check Philip undertook an expedition against the Thracians; but meantime his partisans procured for him an opportunity of marching again into the very heart of Greece. Amphissa, a Locrian town, having been declared by the Amphictyonic council guilty of sacrilege, Philip was appointed by the council as their general to inflict punishment on the inhabitants of the guilty town. Accordingly he marched southwards early in B.C. 338; but instead of proceeding in the direction of Amphissa, he suddenly seized Elatea, the chief town in the eastern part of Phocis, thus showing clearly enough that his real design was against Boeotia and Attica. Intelligence of this event reached Athens at night, and caused extraordinary alarm, In the following morning Demosthenes pressed upon the assembly the necessity for making the most vigorous preparations for defence, and especially recommended them to send an embassy to Thebes, in order to persuade the Thebans to unite with them against the common enemy.