The Celtiberians were the most influential ethnic group in Iberia when the Mediterranean powers (Carthage and Rome) started their conquests. In 220 BC, the Punic army was attacked when preparing to cross the Tagus river by a coalition of Vaccei, Carpetani and Olcades. Despite these clashes, during the Second Punic War the Celtiberians served most often as allies or mercenaries of Carthage in its conflict with Rome, and crossed the Alps in the mixed forces under Hannibal's command. Under Scipio Africanus, the Romans were able to secure alliances and change the allegiances of many Celtiberian tribes, using these allied warriors against the Carthaginian forces and allies in Spain. After the conflict, Rome took possession of the Punic empire in Spain, and some Celtiberians soon challenged the new dominant power that loomed in the borders of its territory. Tiberius Sempronius Gracchus spent the years 182 to 179 pacifying the Celtiberians. Gracchus boasted of destroying over 300 Celtiberian settlements.