On the fringes of the Greek world, Macedonia's people spoke a form of Greek, but the country had different customs and social organization. Macedonia had not followed its southern neighbors in the evolution of the polis, but had retained a chiefdom form of society in which local headmen still wielded considerable power. In a period of twenty-five years, however, Macedonia became the largest empire yet in antiquity, solely as a result of the genius of Philip II and his son, Alexander. A man of exceptional energy, diplomatic skill, and ruthlessness, Philip totally reformed the Macedonian army when he came to power in 359 B.C. Wielding this new weapon, he conquered all the peoples of the southern Balkans, culminating in the defeat of Athens and Thebes in 338. As he was planning to invade Asia, Philip was assassinated in 336. The task of expanding the empire eastward was left to his son.