Battle of Chaeronea (338 BC)
Background: In the decade following his accession in 359 BC, the Macedonian king, Philip II, had rapidly strengthened and expanded his kingdom into Thrace and Chalkidiki on the northern coast of the Aegean Sea. He was aided in this process by the distraction of Athens and Thebes, the two most powerful city-states in Greece at that point, by events elsewhere. In particular, these events included the Social War between Athens and her erstwhile allies (357–355 BC), and the Third Sacred War which erupted in central Greece in 356 BC between the Phocians and the other members of the Delphic Amphictyonic League. Much of Philip's expansion during this period was at the nominal expense of the Athenians, who considered the north Aegean coast as their sphere of influence, and Philip was at war with Athens from 356–346 BC.
Philip was not originally a belligerent in the Sacred War, but became involved at the request of the Thessalians. Seeing an opportunity to expand his influence, Philip obliged, and in 353 or 352 BC won a decisive victory over the Phocians at the Battle of Crocus Field in Thessaly. In the aftermath, Philip was made archon of Thessaly, which gave him control of the levies and revenues of the Thessalian Confederation, thereby greatly increasing his power. However, Philip did not intervene further in the Sacred War until 346 BC.
Early in 346 BC, the Thebans, who had borne the brunt of the Sacred War, together with the Thessalians, asked Philip to assume the "leadership of Greece" and join them in fighting the Phocians. Philip's power was by now so great that ultimately the Phocians did not even attempt to resist, and instead surrendered to him. Philip was thus able to end a particularly bloody war without any further fighting. Philip allowed the Amphictyonic council the formal responsibility of punishing the Phocians, but ensured that the terms were not overly harsh; nevertheless, the Phocians were expelled from the Amphictyonic League, all their cities were destroyed, and they were resettled in villages of no more than fifty houses.

By 346 BC, the Athenians were war-weary, unable to match Philip's strength, and had begun to contemplate the necessity of making peace. Nevertheless, when it became apparent that Philip would march south that year, the Athenians originally planned to help the Phocians (whom they were allied to) keep Philip out of central Greece, by occupying the pass of Thermopylae, where Philip's superior numbers would be of little benefit. The Athenians had successfully used this tactic to prevent Philip attacking Phocis itself after his victory at Crocus Field.
The occupation of Thermopylae was not only for the benefit of Phocis; excluding Philip from central Greece also prevented him from marching on Athens itself. However, by the end of February, the general Phalaikos was restored to power in Phocis, and he refused to allow the Athenians access to Thermopylae. Suddenly unable to guarantee their own security, the Athenians were forced instead into making peace with Philip. Their peace treaty, known as the Peace of Philocrates, made Athens a reluctant ally of Macedon.
For the Athenians, the treaty had been expedient, but it was never popular. Philip's actions in 346 BC had expanded his influence over all Greece, and although he had brought peace, he had come to be seen as the enemy of the traditional liberty of the city-states. The orator and politician Demosthenes had been a principal architect of the Peace of Philocrates, but almost as soon as it was agreed, he wished to be rid of it. Over the next few years, Demosthenes became leader of the "war-party" in Athens, and at every opportunity he sought to undermine the peace. From 343 BC onwards, in order to try to disrupt the peace, Demosthenes and his followers used every expedition and action of Philip to argue that he was breaking the peace. 
Conversely, there was at first a substantial body of feeling in Athens, led by Aeschines, that the peace, unpopular though it was, should be maintained and developed. Towards the end of the decade however, the "war party" gained the ascendancy, and began to openly goad Philip; in 341 BC for instance, the Athenian general Diopeithes ravaged the territory of Philip's ally Cardia, even though Philip demanded that they desist. Philip's patience finally ran out when the Athenians formed an alliance with Byzantium, which Philip was at that time besieging, and he wrote the Athenians declaring war. Shortly afterward Philip broke off the siege of Byzantium; Cawkwell suggests that Philip had decided to deal with Athens once and for all. Philip went on campaign against the Scythians, and then began to prepare for war in southern Greece.

Prelude: Philip's forthcoming campaign in southern Greece became linked with a new, fourth, Sacred War. The citizens of Amphissa in Ozolian Locris had begun cultivating land sacred to Apollo on the Crisaean Plain south of Delphi; after some internal bickering the Amphictyonic council decided to declare a sacred war against Amphissa. A Thessalian delegate proposed that Philip should be made leader of the Amphictyonic effort, which therefore gave Philip a pretext to campaign in southern Greece; it is, however, probable that Philip would have gone ahead with his campaign anyway.
At the start of 339 BC, the Thebans had seized the town of Nicaea near Thermopylae, which Philip had garrisoned in 346 BC. Philip does not appear to have treated this as a declaration of war, but it nevertheless presented him with a significant problem, blocking the main route. However, a second route into central Greece was available, leading over the shoulder of Mount Callidromos and descending into Phocis. However, the Athenians and Thebans had either forgotten the existence of this road, or believed that Philip would not use it; the subsequent failure to guard this road allowed Philip to slip into central Greece unhindered. 
Philip's relatively lenient treatment of the Phocians at the end of the Third Sacred War in 346 BC now bore fruit. Reaching Elatea, he ordered the city to be re-populated, and during the next few months the whole Phocian Confederation was restored to its former state. This provided Philip with a base in Greece, and new, grateful allies in the Phocians. Philip probably arrived in Phocis in November 339 BC, but the Battle of Chaeronea did not occur until August 338 BC. 
During this period Philip discharged his responsibility to the Amphicytonic council by settling the situation in Amphissa. He tricked a force of 10,000 mercenaries who were guarding the road from Phocis to Amphissa into abandoning their posts, then took Amphissa and expelled its citizens, turning it over to Delphi. He probably also engaged in diplomatic attempts to avoid further conflict in Greece, although if so, he was unsuccessful.
When news first arrived that Philip was in Elatea, just three days march away, there was panic in Athens. In what Cawkwell describes as his proudest moment, Demosthenes alone counseled against despair, and proposed that the Athenians should seek an alliance with the Thebans; his decree was passed, and he was sent as ambassador. Philip had also sent an embassy to Thebes, requesting that the Thebans join him, or at least allow him to pass through Boeotia unhindered. Since the Thebans were still not formally at war with Philip, they could have avoided the conflict altogether. However, in spite of Philip's proximity, and their traditional enmity with Athens, they chose to ally with the Athenians, in the cause of liberty for Greece. The Athenian army had already pre-emptively been sent in the direction of Boeotia, and was therefore able to join the Thebans within days of the alliance being agreed.
The details of the campaign leading up to Chaeronea are almost completely unknown.  Philip was presumably prevented from entering Boeotia by way of Mount Helicon, as the Spartans had done in the run-up to the Battle of Leuctra; or by any of the other mountain passes that led into Boeotia from Phocis. There were certainly some preliminary skirmishes; Demosthenes alludes to a "winter battle" and "battle on the river" in his speeches, but no other details are preserved. 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.