Now that he had become tyrant through the folly of the people, Dionysius fought the Carthaginians with no more success than the generals whom he had accused of treason. He was able to save neither Gela nor Camarina, and the entire population of these two towns sought refuge in Syracuse. Displeased by these defeats, the Syracusans tried, but all too late, to rise against him. Supported by his mercenaries, he stifled the rebellion, caused some of his enemies to be put to death, drove the others from the town, and maintained his power by fear. A plague stopped the advance of the Carthaginians and induced them to make peace, but they kept all their conquests, that is to say, more than two-thirds of Sicily, in exchange for a clause of the treaty recognising Dionysius as tyrant of Syracuse. He fortified the isle of Ortygia, of which he made a citadel, after driving out the inhabitants so as to make room for his mercenaries. Then he gave the best part of the Syracusan territory to his friends and to the magistrates; the rest was distributed in equal shares between the citizens, the freed slaves and resident foreigners. This alteration of property caused a rebellion; he shut himself up in his fortress of Ortygia and his mercenaries re-established his authority. Some days later, while the inhabitants were in the fields, busy gathering in the harvest, he had all the houses searched and all weapons removed. When he believed himself absolute master of Syracuse, he wished to extend his rule over the whole of the eastern coast of Sicily. He seized Ætna and Enna, destroyed Naxos and Catana which had been delivered to him by traitors, and sold their inhabitants in order to give their land to the Sicels of the surrounding country and to his Campanian mercenaries. The terrified Leontines opened their gates to him, and were carried to Syracuse. The Rhegians, uneasy at his advance, sent an army into Sicily; but, abandoned by the Messenians, who had at first joined them, they made peace with Dionysius and returned to Italy.
In the meanwhile Dionysius was preparing to revenge himself on the Carthaginians. Syracuse was surrounded by ramparts which made it impregnable. Workmen from all the neighbouring countries, attracted by lure of high wages, were employed to make large supplies of arms and implements of war; it was at this time that the catapult was invented to cast stones and arrows. Numerous warships were built, some of them on a new model with four or five benches of rowers. When these preparations were completed, and mercenaries collected from all sides, Dionysius declared war on the Carthaginians, and, at the head of an army of eighty thousand men, successively re-captured all the towns which they had conquered seven years previously, Gela, Camarina, Agrigentum, Selinus, and Himera, besieged their principal fortress in the isle of Motya on the western point of Sicily, and took it by means of his implements of war (397). But the following year, Himilco landed at Panormus with one hundred thousand men, regained Motya and all the conquests of Dionysius, destroyed Messana, and after a naval victory in sight of Catana, besieged Syracuse by land and sea. Dionysius was obliged to restore to the citizens the arms which he had taken from them, and soon signs of rebellion were again perceived. But once more plague broke out in the Carthaginian army. Himilco paid three hundred talents [£60,000 or $300,000] for permission to withdraw with the Carthaginian citizens who were in his army, abandoning all his mercenaries who were taken and sold as slaves. Hostilities continued for two years longer and the Carthaginians finally made peace by giving up Tauromenium (392).