The Battle of Lake Trasimene was fought when a Carthaginian force under Hannibal ambushed a Roman army commanded by Gaius Flaminius on 21 June 217 BC, during the Second Punic War. The battle took place on the north shore of Lake Trasimene, to the south of Cortona, and resulted in a heavy defeat for the Romans.

War had broken out between Rome and Carthage early in 218 BC. Hannibal, ruler of the Carthaginian territories in south-east Iberia, marched an army through Gaul, crossed the Alps and arrived in Cisalpine Gaul (northern Italy) later that year. The Romans rushed reinforcements north from Sicily but were badly defeated at the Battle of the Trebia.

The following spring, the Romans positioned an army on each side of the Apennine Mountains, but were surprised when a Carthaginian army more than 50,000 strong crossed the range by a difficult but unguarded route. The Carthaginians moved south into Etruria, plundering, razing the villages and killing all men encountered. Flaminius, in charge of the nearest Roman army, set off in pursuit. Hannibal arranged an ambush on the north shore of Lake Trasimene and trapped the Romans. With the Carthaginians attacking unexpectedly from the flank and the rear, possibly in poor visibility, there was no chance for the Romans to form even a rudimentary fighting line and they were defeated after three hours of hard fighting with 15,000 being killed. The trap failed to enclose the 6,000 Romans at the front of the column, who escaped; later in the day they were surrounded by pursuing Carthaginians and surrendered. Thus nearly all 25,000 Romans in Flaminius's army were killed or captured. This destruction of an entire army as a result of an ambush by another army is widely considered a unique occurrence. Several days later the Carthaginians wiped out the entire cavalry force of the second Roman army, who were not yet aware of the earlier disaster.

The Carthaginians then marched towards southern Italy in the hope of winning over some of the ethnic Greek and Italic city-states there. News of the defeat caused a panic in Rome and led to the election of Quintus Fabius Maximus Verrucosus as dictator. Impatient with his Fabian strategy of avoiding major battles, the next year the Romans elected Lucius Paullus and Gaius Varro as consuls. These more aggressive commanders engaged Hannibal at the Battle of Cannae in 216 BC, resulting in a third and even worse disaster for Rome; it was followed by thirteen more years of war.

The First Punic War was fought from 264 to 241 BC between Carthage and Rome: these two main powers of the western Mediterranean in the 3rd century BC struggled for supremacy primarily on the Mediterranean island of Sicily and its surrounding waters and in North Africa. The war lasted for 23 years until the Carthaginians were defeated. Five years later an army commanded by the leading Carthaginian general Hamilcar Barca landed in Carthaginian Iberia (modern south-east Spain) which he greatly expanded and turned into a quasi-monarchical, autonomous territory ruled by the Barcids. This expansion gained Carthage silver mines, agricultural wealth, manpower, military facilities such as shipyards and territorial depth, which encouraged it to resist future Roman demands.

Hamilcar ruled as viceroy until his death in 228 BC. He was succeeded by his son-in-law, Hasdrubal, then his son Hannibal in 221 BC. In 226 BC the Ebro Treaty established the Ebro River as the northern boundary of the Carthaginian sphere of influence in Iberia. A little later Rome made a separate treaty of association with the independent city of Saguntum, well south of the Ebro. In 219 BC a Carthaginian army under Hannibal besieged, captured and sacked Saguntum, which led Rome to declare war on Carthage.

Meanwhile, the major Gallic tribes in Cisalpine Gaul (modern northern Italy), antagonised by the founding of several Roman settlements on traditionally Gallic territory, attacked the Romans, capturing several towns and repeatedly ambushing a Roman relief force. The Roman Senate reinforced its army in Cisalpine Gaul while also preparing armies to invade Carthaginian territories.

Hannibal assembled a Carthaginian army in New Carthage (modern Cartagena) and marched north into Gaul in May 218 BC, leaving his brother, Hasdrubal Barca, in charge of Carthaginian interests in Iberia. The Carthaginian army crossed the Alps in October, surmounting the difficulties of climate, terrain and the guerrilla tactics of the native tribes. Hannibal arrived with 20,000 infantry, 6,000 cavalry and an unknown number of war elephants – the survivors of the 37 with which he had left Iberia – in Cisalpine Gaul. The Romans had already withdrawn to their winter quarters and were astonished by Hannibal's appearance.