The next couple of years are poorly documented, which suggests that Philip was involved in the northern areas of his kingdom. In 345, he was active in the north of what is now Albania (receiving a wound in his leg); he continued to reorganize Thessaly, and in late 342, we find the Macedonian army in Thrace, where it subdued the kingdom of a man named Cersobleptes, and from where Philip continued to the Black Sea. In 341, he founded Philippopolis, modern Plovdiv. The area between the Aegean Sea and the Balkan mountains received a viceroy, not unlike a Persian satrap.
The obvious next step was to expand the frontiers to the Sea of Marmara, where several Greek cities were still independent. In the winter of 341/340, Philip started to besiege Perinthus and Byzantium. Then, the unthinkable happened: Artaxerxes III Ochus ordered direct Persian intervention in Europe, something that had not happened since the days of his ancestor Xerxes. The shock that the Macedonians experienced was still felt eight years later, when Philip's son Alexander wrote a letter to the Persian king Darius III Codomannus, in which he stated that the Persian help to Perinthus and Byzantium had been the cause of war (text).
But although this may have been unexpected to Philip, in fact, the Persian king had no alternative. The Macedonians threatened the use of the straits, one of Persia's vital interests. Until then, the normal policy of Persian kings against the Yaunâ (Greeks) had been to set up others against them. But now, Philip was allied to all Greeks, so there was no alternative for direct intervention. Therefore, Artaxerxes ordered satrap Arsites of Hellespontine Phrygia and general Mentor of Rhodes to send mercenaries to Perinthus.
It seems that Philip first wanted to continue the struggle. Back home, prince Alexander was appointed as regent, and Philip ordered his trusted general Parmenion to come to his assistance. At the same time, the Persians invaded Thrace and the satrap of Caria ordered the Greeks of Rhodes, Cos, and Chios to send troops to Perinthus. The Athenians were in a state of shock, because they imported food from the Black Sea area, and could not allow Philip to obtain a stronghold in Perinthus or Byzantium, where he could cut off the Athenian food supply. When Philip did indeed seize 240 grain ships, they declared war. Now, the Macedonian king had to give up the siege of Perinthus. It was the second defeat in his career.
But Europe had never seen a man like king Philip of Macedonia, who possessed the talent to benefit from his very defeats. He accepted the loss of Perinthus as the price he had to pay for something better: war in the south, where he had already broken the powers of the Greek cities. He only needed to conquer them, so that he could reorganize Greece according to his wishes. This would be easy, because during the last decades, the Greeks had relied upon mercenaries, but after the end of the Third Sacred War, these professionals had been hired by the Persians. The Greeks could only employ citizen levies, whereas the Macedonian king would use a well-trained, professional army.
Philip was not in a hurry. In 339, he campaigned north of the Balkan mountains, near the Danube, and it was only in 338 that he finally descended upon Greece. In the meantime, the Athenian politician Demosthenes had created an anti-Macedonian alliance, and it looks as if Philip postponed to strike until it was ready. He wanted one, decisive battle against a well-prepared army, to make it very clear that the Greeks were no match. The final battle took place in August 338, near Chaeronea, west of Thebes (text). Philip employed less than half of his army, and his victory was never in doubt.
Now, everybody knew that the Greeks had not recuperated from the Third Sacred War. They had had all time to prepare themselves and had been decisively beaten. The Macedonian propaganda mad sure that everyone understood the significance of this battle. Philip had conquered Greece. And besides, much was made of the fact that crown prince Alexander, eighteen years old, had led the decisive charge. Philip's dynasty was strong. Resistance was futile.