tyrant of Syracuse from 405 who, by his conquests in Sicily and southern Italy, made Syracuse the most powerful Greek city west of mainland Greece. Although he saved Greek Sicily from conquest by Carthage, his brutal military despotism harmed the cause of Hellenism.

After working as a clerk in a public office, Dionysius distinguished himself fighting in the war with Carthage that broke out in Sicily in 409. He took advantage of a crisis in the war to make himself tyrant in 405. Over the next eight years he ruthlessly consolidated and expanded his power. He built a wall around Syracuse and fortified Epipolae. The Greek citizens of Naxos, Catana, and Leontini were removed from their cities; many of them were enslaved and their homes were given to Sicilian and Italian mercenaries.

Dionysius was then ready to lead his vast army against Carthage, which had occupied western and southern Sicily. His first war with Carthage (397–396), during which the Greeks besieged Motya and the Carthaginians Syracuse, ended with a notable victory for Dionysius, who confined his enemy's power to an area of northwest Sicily. A second conflict ended in 392 with a treaty advantageous to Dionysius. After 390 he led an expedition against Rhegium and other Greek cities of southern Italy, and with the aid of the Lucanians he devastated the territories of Thurii, Croton, and Locri. By the time Rhegium fell (386), Dionysius had become the chief power in Greek Italy. He sent colonists to Illyria and possibly to northeast Italy. Although the Athenian writer Isocrates hailed him as a champion of Hellenism, the brutality of Dionysius' conquests made him unpopular in Greece, and his literary pretensions were deplored. When he sent a splendid embassy to the Olympic festival of 388, a crowd pillaged the tents of his envoys.

Dionysius' third war with Carthage (383–c. 375) proved disastrous; he suffered a crushing defeat at Cronium and was forced to pay an indemnity of 1,000 talents and cede the territory west of the Halycus River. Nevertheless, he was engaged in yet another conflict with the Carthaginians at the time of his death.