The war began with the Carthaginian generalHannibal's conquest and destruction of the Roman-allied Iberian city of Saguntum in 219 BC, prompting a Roman declaration of war on Carthage in the spring of 218. Hannibal surprised the Romans by marching his army overland from Iberia to cross the Alps and invade Roman Italy, followed by his reinforcement by Gallic allies and crushing victories over Roman armies at Trebia in 218 and on the shores of Lake Trasimene in 217. Moving to southern Italy in 216, Hannibal at Cannaeannihilated the largest army the Romans had ever assembled, killing or capturing more than 67,000 Roman soldiers. After the death or imprisonment of 130,000 Roman troops in two years, 40% of Rome's Italian alliesdefected to Carthage, giving her control over most of southern Italy. Macedon and Syracusejoined the Carthaginian side after Cannae and the conflict spread to Greece and Sicily. The Carthaginian navy was built up in the early years of the war, and from 215–210 the Carthaginian army and navy launched amphibious assaults with over 50,000 ground troops to capture Roman Sicily and Sardinia but were ultimately repulsed.
Against Hannibal's skill on the battlefield, the Romans adopted the Fabian strategy - the avoidance of battle against Hannibal and defeating his allies and the other Carthaginian generals instead. Roman armies recaptured all of the great cities that had joined Carthage and defeated a Carthaginian attempt to reinforce Hannibal at Metaurus in 207. Southern Italy was devastated by the combatants, with hundreds of thousands of civilians killed or enslaved. In Iberia, which served as a major source of silver and manpower for the Carthaginian army, a Roman expeditionary force under Publius Cornelius ScipiocapturedCarthago Nova, Carthage's capital city in Iberia, in 209, massacring and enslaving the inhabitants. Scipio's destruction of a Carthaginian army at Ilipa in 206 permanently ended Carthaginian rule in Iberia. He invaded Carthaginian Africa in 204, inflicting two severe defeats on Carthage and her allies at Utica and the Great Plains that compelled the Carthaginian senate to recall Hannibal's army from Italy. The final engagement between Scipio and Hannibal took place at Zama in Africa in 202 and resulted in Hannibal's defeat and the imposition of harsh peace conditions on Carthage (Carthaginian peace), which ceased to be a great power and became a Roman client state until its final destruction by the Romans in 146 BC during the Third Punic War. The Second Punic War overthrew the established balance of power of the ancient world and Rome rose to become the dominant power in the Mediterranean Basin for the next 600 years.
Carthage's defeat in the First Punic War meant the loss of Carthaginian Sicily to Rome under the terms of the Roman-dictated 241 BC Treaty of Lutatius. Rome exploited Carthage's distraction during the Truceless War against rebellious mercenaries and Libyan subjects to break the peace treaty and annex Carthaginian Sardinia and Corsica to Rome in 238 BC. Under the leadership of Hamilcar Barca and his family, Carthage defeated the rebels and began the Barcid conquest of Hispania from 237 BC onward. Control over Spain gave Carthage the silver mines, agricultural wealth, manpower, military facilities such as shipyards and territorial depth to stand up to future Roman demands with confidence. The highest priority in Carthaginian strategy, apart from the defeat of Rome, was to keep the war away from Carthage's agricultural heartland in Africa and protect the property of the wealthy Carthaginian landowners who controlled Carthaginian politics.
The Second Punic War was ignited by the dispute over the hegemony of Saguntum, a Hellenized Iberian coastal city with diplomatic contacts with Rome. After great tension within the city government, culminating in the assassination of the supporters of Carthage, Hannibal laid siege to the city of Saguntum in 219 BC. The city called for Roman aid, but the pleas fell on deaf ears. Following a prolonged siege of eight months and a bloody struggle, in which Hannibal himself was wounded, the Carthaginians finally took control of the city. Many of the Saguntians chose to commit suicide rather than face subjugation by the Carthaginians. The loss of Saguntum as a potential base of operations in Carthaginian Iberia was a serious setback to the main Roman strategic objective in Spain: the eviction of the Carthaginians from the peninsula. The Roman Senate sent an embassy to the Carthaginian Senate that declared war on Carthage in early 218 BC over the attack on Rome's Saguntine ally.
Before the war, Rome and Hasdrubal the Fair had made a treaty.Livy reports that it was agreed that the Iber should be the boundary between the two empires and that the liberty of the Saguntines should be preserved.:21.13