Xelloss wrote:Hannibal's crossing of the Alps is a strong contender
Feel obliged to counter this due living in what was once the one Roman stronghold in Northern Italy that resisted Hannibal's invasion.
The situation at the start of the war was more or less this: Italy was almost completely under Roman control due a network of alliances (mostly with the Greek cities) and varying levels of more or less willing subjugation and cultural assimilations (for example the Etruscans and Latins were by then rather loyal to Rome and Romans in almost anything but the names, while the Samnites and Gauls south of the Po river were ready to rebel at a moment's notice if they thought they could win), and, together with Sicily (that was not considered part of Italy at the time), provided Rome with greater manpower and finances than Carthage could get from its African territories and Spain; the 'free' Italians, the Gauls on the banks of the Po river and north of it, were too in alliance with Rome, albeit on more vague and strict terms than other peoples, and Rome had started a program of expansion in their lands and Romanization of the Gauls (including the creation of fortified strongholds at Placentia and Cremona with decent-sized garrisons); Spain provided Carthage with numerous soldiers and money, but was nowhere under control as Italy was for Rome nor as rich; finally, Rome controlled both the sea and, in modern day France, Massilia (a Greek city under Roman protection), blocking the best ways to invade Italy.
Hannibal had recognized that Rome's strength came from control of Italy, and the only way to break such control was to inflict crushing defeats on the Romans on Italian soil, thus causing rebellions first among the Gauls and Samnites and then among the Roman loyalists until the Romans were overran. To attack Italy, he had three possible routes: the sea, that was controlled by the Roman fleet; the area of Nice, that was where the Romans waited for him after going to Massilia by sea, and was presided by pro-Roman Ligurians and their fortress at Genua; and the Alps, where there was no Roman legion to wait him and was inhabitated only by the pro-Roman but relatively weak Ligurian tribe of the Taurini and the stronger anti-Roman Insubres (a Gaulish tribe).
Knowing this, passing the Alps was the best way to invade Italy: crossing the sea was nearly suicidal due the supremacy of the Roman Navy; passing from Nice exposed him to a battle against Scipio's (father of the Africanus) fresh Roman army and, even if victorious, would have forced force him to enter hostile territory and try and storm or besiege a fortified city while another Roman army (possibly reinforced by Cremona and Placentia) marched against him; passing the Alps killed many of his elephants, but allowed him to enter a partially favourable territory, join his troops with powerful allies, knock out of the war one of Rome's allies (the Taurini, whose main or only town was razed) and face Scipio's army tired and weakened by sending parts of the troops to attack Spain. And, by defeating Scipio at the Ticinus river, caused a mass defection among the Gauls allied more or less willingly to Rome and many of the Ligurians to join him.
By all senses, Hannibal should have won the war in 217 A.C., after the annihilation of a Roman army at the lake Transimene, or after the even greater victory at Cannae, and would have had his strategy be even a little more successful or the Romans done what everyone else would have done after such a disaster and sued for peace.
After Cannae, the situation was the following:
Spain was contested, and could send little to no reinforcements;
Carthage's attempt to reinforce Hannibal by sea had to run the blockade of the Roman Navy, that was still intact and much stronger than Carthage's;
Northern Italy was pretty much under Hannibal's allies, with pro-Roman Ligurians overran and Genua destroyed (Placentia would follow in 209, but by then the Romans were counterattacking);
Southern Italy and Sicily were divided, with many of the Greek cities and part of the Souther Italians defecting (most importantly Cuma and the Samnites, then the only possible rivals to Roman hegemony in Italy), but the Greek cities of the Tyrrhenian coast (rivals of Cuma and the campanian peoples) and other in the rest of the territory, the Apulians, and even part of the Samnites remained on Rome's side, with the continued loyalty of Neapolis (the biggest seaport of Campania) and Rhegium (that controlled one of the shores of the Strait of Messina and was literally sitting on Hannibal's communication line with Sicily) being the biggest thorn in his flank and many of the defecting cities being divided between pro-Romans and pro-Carthaginians and liable to switch side;
in Central Italy, the southernmost Gauls had joined Hannibal, but the Etruscans, the Umbrian-speaking tribes and the Latins had stayed loyal, and they commanded most of Italy's manpower and riches, controlled the area around Rome (and the Umbrians had previously foiled Hannibal's initial attempt to attack Rome after the Transimene), and made communications and linking with his northern forces difficult;
total manpower of Rome and the loyalist is estimated in about 500,000 people (minus the 100,000 lost in Hannibal's initial triumphs), fully integrated in the Roman command structure and available for deployment anywhere, while the defectors could muster about 150,000 people, not fully trusted by Hannibal (with the exception of Hannibal's Gaulish veterans that joined him at the start of the campaign), mostly serving in their own units, and, with the exception of Gaulish veterans and Lucani, not available outside of their home territories;
Massilia is mostly a non-factor, as it has enough trouble on its own dealing with the Gauls, but can at least hold down some of the rebelling Ligurians;
almost forgot, Rome's control of the sea meant they could attack Sicily and pick Hannibal's allies there one by one.
As far as I know, Rome's strength was severely tested, and it's almost a miracle they actually resisted long enough to pick off Hannibal's allies, reconquer Northern Italy, conquer Spain and weaken Hannibal's army enough to make it defeatable.