Battle of Chaeronea (338 BC)
Finally, in August 338 BC, Philip's army marched straight down the main road from Phocis to Boeotia, to assault the main allied army defending the road at Chaeronea.

Opposing forces: According to Diodorus, the Macedonian army numbered roughly 30,000 infantry and 2,000 cavalry, a figure generally accepted by modern historians. Philip took command of the right wing of the Macedonian army and placed his 18-year-old son Alexander (the future conqueror of the Persian Empire) in command of the left wing, accompanied by a group of Philip's experienced generals.
The allied army included contingents from Achaea, Corinth, Chalcis, Epidaurus, Megara, and Troezen, with the majority of troops being supplied by Athens and Thebes. The Athenian contingent was led by the generals Chares and Lysicles, and the Thebans by Theagenes. No source provides exact numbers for the allied army, although Justin suggests that the southern Greeks were "far superior in number of soldiers". The modern view is that the numbers of the city states that fought were approximately equal to those of the Macedonians. 
The Athenians took up positions on the left wing, the Thebans on the right, and the other allies in the centre. Athenian aristocrat Philippides of Paiania, campaigned for Philip's cause with his associates during the battle in Thebes, Eleuthera and Tanagra. Philippides was later prosecuted by Hypereides for his pro-Macedonian actions after the defeat.

Strategic and tactical considerations: The allied army of Athens and Thebes had taken up a position near Chaeronea, astride the main road. On the left flank, the allied line lay across the foothills of Mount Thurion, blocking the side-road that led to Lebedea, while on the right, the line rested against the Kephisos River, near a projecting spur of Mount Aktion. The allied line, which was about 2.5 miles in length, was thus secure on both flanks. Moreover, the allied line seems to have slanted north-eastwards across the plain in between, so that it did not face the direction of Macedonian advance full-square. 
This prevented Philip from attempting to concentrate his force on the allied right wing, since the advanced position of the left wing would then threaten Philip's right. Although Philip could attempt to concentrate his force against the southern Greek left, the troops there occupied high ground, and any attack would be difficult. Since the southern Greeks could remain on the defensive, having only to prevent Philip's advance, their position was therefore strategically and tactically very strong.

Battle: Details of the battle itself are scarce, with Diodorus providing the only formal account. He says that "once joined, the battle was hotly contested for a long time and many fell on both sides, so that for a while the struggle permitted hopes of victory to both." He then recounts that the young Alexander, "his heart set on showing his father his prowess" succeeded in rupturing the allied line aided by his companions, and eventually put the right wing to flight; meanwhile, Philip advanced in person against the left wing and also put it to flight.
This brief account can be filled out, if Polyaenus's account of the battle is to be believed. Polyaenus collected many snippets of information on warfare in his Strategems; some are known from other sources to be reliable, while others are demonstrably false. In the absence of other evidence, it is unclear whether his passage regarding Chaeronea is to be accepted or rejected. Polyaenus suggests that Philip engaged the allied left, but then withdrew his troops; the Athenians on the left followed and, when Philip held the high ground, he stopped retreating and attacked the Athenians, eventually routing them. 
In another 'stratagem', Polyaenus suggests that Philip deliberately prolonged the battle, to take advantage of the rawness of the Athenian troops (his own veterans being more used to fatigue) and delayed his main attack until the Athenians were exhausted. This latter anecdote also appears in the earlier Stratagems of Frontinus.
Polyaenus's accounts have led some modern historians to tentatively propose the following synthesis of the battle. After the general engagement had been in progress for some time, Philip had his army perform a wheeling manoeuver, with the right wing withdrawing, and the whole line pivoting around its centre. At the same time, wheeling forward, the Macedonian left wing attacked the Thebans on their right and punched a hole in the allied line. On the allied left, the Athenians followed Philip, their line becoming stretched and disordered; the Macedonians then turned, attacked and routed the tired and inexperienced Athenians. The southern Greek right wing, under the assault of the Macedonian troops under Alexander's command, then attacked, ending the battle.
Many historians, including Hammond and Cawkwell, place Alexander in charge of the Companion Cavalry during the battle, perhaps because of Diodorus's use of the word "companions". However, there is no mention of cavalry in any ancient account of the battle, nor does there seem to have been space for it to operate against the flank of the southern Greek army. However, some Macedonian cavalry squadrons in wedged formation, a tactical deployment Philip designed for his cavalry for just such a purpose, could have penetrated the gaps which opened amid the Greek lines, gaps which are mentioned by Diodorus in his brief account. Plutarch says that Alexander was the "first to break the ranks of the Sacred Band of the Thebans", the elite of the Theban infantry, who were stationed on the extreme right of the battle line. However, he also says that the Sacred Band had "met the spears of [the Macedonian] phalanx face to face". This, together with the improbability that a head-on cavalry charge against the spear-armed Thebans could have succeeded, has led Gaebel and others to suggest that Alexander must have been commanding a portion of the Macedonian phalanx at Chaeronea.
Diodorus says that more than 1,000 Athenians died in the battle, with another 2,000 taken prisoner, and that the Thebans fared similarly. Plutarch suggests that all 300 of the Sacred Band were killed at the battle, having previously been seen as invincible. In the Roman period, the 'Lion of Chaeronea', an enigmatic monument on the site of the battle, was believed to mark the resting place of the Sacred Band. Modern excavations found the remains of 254 soldiers underneath the monument; it is therefore generally accepted that this was indeed the grave of the Sacred Band, since it is unlikely that every member was killed.

Aftermath: Cawkwell suggests that this was one of the most decisive battles in ancient history. Since there was now no army which could prevent Philip's advance, the war effectively ended. In Athens and Corinth, records show desperate attempts to re-build the city walls, as they prepared for siege. However, Philip had no intention of besieging any city, nor indeed of conquering it. He wanted the southern Greeks as his allies for his planned campaign against the Persians, and he wanted to leave a stable Greece in his rear when he went on campaign; further fighting was therefore contrary to his aims. Philip marched first to Thebes, which surrendered to him; he expelled the Theban leaders who had opposed him, recalled those pro-Macedonian Thebans who had previously been exiled, and installed a Macedonian garrison. He also ordered that the Boeotian cities of Plataea and Thespiae, which Thebes had destroyed in previous conflicts, be re-founded. Generally, Philip treated the Thebans severely, making them pay for the return of their prisoners, and even to bury their dead; he did not, however, dissolve the Boeotian Confederacy.
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.