Except in politically backward or thinly populated areas Carthage's foreign policy was non-expansive. One major departure from this policy - Sicily - was initially a disaster.

In 480 Carthage, trusting in a powerful navy, intervened in inter-city struggles among the Greeks of Sicily. At the battle of Himera off the Sicilian coast, their fleet suffered disastrous defeat by Gelon of Syracuse and his allies. Carthage was forced to curb against its appetite for expansion overseas, althuogh it continued to enjoy remarkable economic prosperity.

Carthage had expanded its domain well beyond its seat on the north coast of Africa, and by 409 BC was ready to renew its adventures against the Greeks of Sicily. Confusing hostilities ensued in see-saw fashion for over a century. After a long period of peace, it went in 410 to the help of Segesta, an ally in Sicily, and turned the war into one of revenge for the earlier defeat. The wars opened with a lightning-fast three months invasion by Carthage. In addition to the Phonician settlements in northwest Sicily that were her traditional allies, Carthage was further strengthened by an alliance with Segesta. The allied forces first stormed and destroyed the coastal Sicilian city of Selinunte [Selinus], Segesta's long-time enemy.

Then the Carthaginian forces sped across Sicily to Himera on the northern coast, site of their overwhelming defeat just 69 years earlier. Led by Syracuse, other Greek cities of the island hurridly sent support to Himera, but the Sicel and Siculi settlements sided with the invaders. After seizing Himera in a furious assault and slaughtering its inhabitants, the victorious Carthaginian leader returned to Carthage, leaving his forces in firm control of the entire area to the north and west of the captured cities. During the 4th century most of the region's wars were caused by the attempts of various rulers of Syracuse to drive the Carthaginians out of Sicily; three of these (398-392, 382-375, and 368) were with Dionysius the Elder of Syracuse.

Three years later the Carthaginians attacked again, this time seizing and sacking the major stronghold at Agrigento [Acragas]. The invaders then turned their sights on Gela.

Syracuse, now emerging under the leadership of Dionysius from a period of political instability, moved to intervene. Dionysius' army soon withdrew in defeat and Gela fell as well. Dionysius' enemies, perhaps with justification, accused him of entering a secret arrangement with the Carthaginians, allowing Gela to fall in exchange for a subsequent truce that strenghened his still-shaky hold on power in Syracuse. In any event, the Carthaginians halted their advance and Dionysius was left with control of Syracuse and the remaining Greek area except Messina [Messana] and the Sicel settlements, which became independent. At this point, however, the tide of affairs turned in favor of Syracuse and its allies. By 398 BC Dionysius felt strong enough to launch an attack against one of the Carthaginian strongholds. Carthage reacted strongly and soon had Syracuse itself under siege, but in a sudden reversal the Carthaginians were routed and driven entirely from Sicily. Carthage found that she had lost all she had gained since her original invasion of 409 BC. In further campaigns in the ensuing ten years, Syracuse and her allies repulsed a new attack from Carthage and then moved offensively with a territorial expansion of her own, seizing--temporarily--territory in the "toe" of the Italian mainland across the Strait of Messina.

Most of the time the eastern limit of Carthaginian power in the island was recognized as the Halycus (Platani) River. The only occasion in which Carthage suffered directly (since its armies were largely mercenary) was in 310, when the ruler of Syracuse, Agathocles, under heavy pressure in Sicily, launched a daring invasion of Africa, the first experienced by Carthage. Over a period of three years he caused great devastation in Carthaginian territory in eastern Tunisia but in the end was defeated.

Finally, a lasting peace was reached between the warring powers in 301 BC that left Carthage entrenched in the Phoenician cities of northwestern Sicily. Ultimately, Carthage gained no real territorial advantage from the century of war, but Syracuse was gravely weakened. The war, combined with a tumultuous succession of domestic rulers, loosened Syracuse's hegemony over the Greek cities of Sicily. Agrigento especially emerged as a serious rival power in southern and central Sicily. Syracuse was reduced to a sphere of dominance in eastern Sicily, and events were set in motion for the appearance of a new and overwhelming player on the scene: Republican Rome.

http://www.boglewood.com/sicily/carthage.html