While these events were taking place in the north, the Third Sacred War had broken out in the south, the greatest disturbance in the history of Greece, and in fact the end of its independence. Thebes had suffered some minor setbacks on Euboea and wanted to compensate for them by the conquest of Phocis, a comparatively powerless state in the west of central Greece. However, the Phocians learned what was about to happen, and in the spring of 356, they helped themselves to the temple treasury of Delphi, and hired an army of mercenary. Technically, this was sacrilege, and it offered the Thebans an excuse to do what they wanted to do anyhow: they were fighting for the honor of the god of Delphi, Apollo.
When the Thebans had learned that Athens had lost the Social War (summer 355), they understood that this once powerful state could not help the Phocians, and in 354, war broke out. Within a few month, all Greek cities were allied to one of the opposing sides. In the east, the former allies of Athens sided with Thebes, and Athens itself, although powerless, joined the Phocians. Sparta did the same, and its archenemies Agros and Megalopolis received help from Thebes; in the far west, the parties in the civil wars of Syracuse and the rest of Sicily and southern Italy lined up with one side or another; and in the north, the inhabitants of Pherae used the Phocians to liberate Thessaly from Theban occupation (or, the other way round, the Thessalians asked Theban help against the Pheraeans).
While the Greeks were destroying each other, two kings were smiling: Artaxerxes III Ochus knew that his Egyptian enemies would receive no support, and Philip knew that he could consolidate his conquests. In 353, he defeated the Thracian leader Amadocus and Cetriporis, who became his vassals. The eastern frontier, which was destabilized after the annexation of Crenides/Philippi, was secure again.
In fact, the future was even brighter than Philip expected. In Thessaly, the inhabitants of Larisa and their Theban allies were challenged by the Pheraeans and the Phocians. Traditionally, Larisa had been allowed to appoint the tagos, president, of the Thessalian confederacy, but the Pheraeans had obtained it under their leader, a tyrant named Jason. But he had been killed and everything was open again. Philip, who hardly needed an invitation, gladly offered help to the Larisans when they requested it in 353, and invaded Thessaly, proclaiming to fight for Thebes and the honor of Apollo, whose portrait from now on graced Macedonian coins.
Unexpectedly, Philip was defeated, for the first time in his life, by the Phocian commander Phayllus, who employed catapults, a weapon that the Macedonians had never seen before. During the winter, Philip had to overcome a mutiny of his soldiers (the only one in his career), but in 352, he was back again. The Phocians and Pheraeans offered battle on the coastal plain (the "Crocus Field"), where they expected Athenian help, but the Macedonian king had already defeated his opponents before the Athenians arrived. Philip ordered the execution of the sacrilegious Phocians, proceeded to Larisa, and got what he wanted: he was the new tagos.
The surrender of Pherae was a matter of time, and although Philip was unable to advance to central Greece (the Athenians occupied Thermopylae), 352 had been a good year for Philip. He had now united Macedonia and Thessaly in a personal union: a spectacular achievement, which gave him membership in the panhellenic organizations. He was no longer considered a barbarian.
This was not the end of Philip's successes. On learning that the Athenians had garrisoned Thermopylae, he did not continue his push to the south, but instead invaded Thrace, where he reached the Sea of Marmara in November 352. It showed the world that his army was faster than any other.
His whereabouts in 351 are unreported, but there was a rumor that he was ill - perhaps because he really was, or perhaps because he was, for once, inactive and preferred to stay at home with his wives and two six-year old sons. Anyhow, in 350, the Olynthians appealed to Athens for help. It seems that Philip had decided to conquer Olynthus and its neighborhood now that no Greek power could help it. In 349, he laid siege to the city, and in the next year, he took it (after he had bought the help of a traitor), and razed it to the ground. In the meantime, he intervened among the Molossians, another former ally, and expelled king Arybbas. The new king was Olympias' brother Alexander of Molossis.
Meanwhile, the Sacred War was still going on. First, Thebes had been successful, then Phocis had recovered and after a period of guerilla warfare, the Phocians invaded Boeotia in 347. Their funds were now running out and this invasion was their last attempt to win the war. Thebes was tired of the war too, and appealed to the Macedonian king. What happened exactly is unclear. There was an attempt to block Thermopylae, but the garrison was not in place when the Macedonian army arrived, and in 346, the Phocians surrendered to Philip. Perhaps, as is stated in some sources, he had bribed the Phocian generals, but this may be contemporary propaganda, broadcasted to demoralize the Phocians. Anyhow, Phocis surrendered, a peace treaty was concluded, and its two votes in the panhellenic councils were given to Philip.
The Third Sacred War was over. Thebes, Phocis, and all other Greek city-states were seriously weakened, and Philip had shown this part of the world that he was its supreme leader. A little earlier, the Macedonian king had already concluded a second treaty, with Athens (the "peace of Philocrates"). This marked the end of a war that had started when Philip had attacked Amphipolis and Pydna in 357 (above).
The historian Diodorus of Sicily states that by now, Philip was already thinking about an invasion of the Achaemenid Empire. This may explain why Philip and Athens allied themselves: the Macedonian army, supported by the Athenian navy, could be a serious enemy for the Persian king Artaxerxes III Ochus, who was still trying to conquer Egypt. Yet, war was not to break out yet, and perhaps the anecdote told by Plutarch is more reliable: Persian envoys visited the Macedonian court, and received a warm welcome from the young prince Alexander (text).