What was the cause of the Third Punic War?

The Third Punic War (149 BC - 146 BC) involved an extended siege of Carthage, ending in the city’s thorough destruction. The resurgence of the struggle can be explained by growing anti-Roman agitations in Hispania and Greece, and the visible improvement of Carthaginian wealth and martial power in the fifty years since the Second War.

With no military, Carthage suffered raids from its neighbour Numidia. Under the terms of the treaty with Rome, such disputes were arbitrated by the Roman Senate. Because Numidia was a favored client state of Rome, Roman rulings were slanted heavily to favor the Numidians. After some fifty years of this condition, Carthage had managed to discharge its war indemnity to Rome, and considered itself no longer bound by the restrictions of the treaty, although Rome believed otherwise. Carthage mustered an army to repel Numidian forces. It immediately lost the war with Numidia, placing itself in debt yet again, this time to Numidia.

This new-found Punic militarism alarmed many Romans, including Cato the Elder who after a voyage to Carthage, ended all his speeches, no matter what the topic, by saying: “Ceterum censeo Carthaginem esse delendam.” - “Furthermore, I think that Carthage must be destroyed”.

In 149 BC, in an attempt to draw Carthage into open conflict, Rome made a series of escalating demands, one being the surrender of three hundred children of the nobility as hostages, and finally ending with the near-impossible demand that the city be demolished and rebuilt away from the coast, deeper into Africa. When the Carthaginians refused this last demand, Rome declared the Third Punic War. Having previously relied on mercenaries to fight their wars for them, the Carthaginians were now forced into a more active role in the defense of their city. They made thousands of makeshift weapons in a short amount of time, even using women’s hair for catapult strings, and were able to hold off an initial Roman attack. A second offensive under the command of Scipio Aemilianus resulted in a three-year siege before he breached the walls, sacked the city, and systematically burned Carthage to the ground in 146 BC. The popular story that the ground was seeded with salt was invented in the nineteenth century as there are no evidence in any ancient sources.

http://en.wikipedia.org/wiki/Punic_War#Third_Punic_War_.28149_BC_to_146_BC.29