Philip (382 - 336 BCE) had spent some years living as a political hostage in Greece (Thebes), but was eventually returned home where he became king. While in Thebes he had learned a great deal about the Greeks, including the equipment and functions of the phalanx, which was by far the most effective military formation/tactic in the world at the time. Philip not only brought this knowledge back with him, but expanded upon it, having the soldiers carry spears (sarrisas) that were twice the length of those used in the traditional phalanx, giving Philip's troops an advantage when he began a campaign of systematic conquest in 356.
One by one the Greek poleis fell before Philip, until he had conquered all of them except for Sparta, although he had stripped Sparta of her Peloponnesian empire, thus robbing her of her power base and the ability to attack. The actual Spartans, however, remained intact, and Philip realized that taking the city itself would be far more costly than it was worth. Philip at one point sent a threatening letter to the Spartans saying "If I enter Laconia," (the region surrounding and including Sparta) "I will level Sparta to the ground." The Spartans, in traditional Spartan boldness and Laconic conciseness, simply responded with a letter reading: "if."
Having conquered Greece, Philip faced the challenge of how to rule it, and more specifically, how to keep them from rebelling. He began planning an invasion of Persia, which would not only be a lucrative venture, but would also be a cause that the Greeks themselves could get behind, considering the long-standing animosity the Greeks held for Persia, having been invaded by Persia several times. The expedition was framed as something as a punitive campaign. However, Philip was never to travel to Persia, as he was assassinated in 336 at a festival, and succeeded by his son Alexander, who took up the campaign of Persian conquest.