When he left his Spanish base one spring day in 218 B.C. with his 100,000-man army of mercenaries, officers, and elephants, Hannibal was launching not just the main offensive of the Second Punic War but also one of the great military journeys in ancient history. His masterful advance through rough terrain and fierce Celtic tribes proved his worth as a leader, but it was his extraordinary passage through the Alps--still considered treacherous even...