In 219BC Hannibal besieged, captured and sacked the pro-Roman city of Saguntum, prompting a Roman declaration of war on Carthage in spring 218BC. That year, Hannibal surprised the Romans by marching his army overland from Iberia, through Gaul and over the Alps to Cisalpine Gaul (modern northern Italy). Reinforced by Gallic allies, he obtained crushing victories over the Romans at the battles of Trebia (218) and Lake Trasimene (217). Moving to southern Italy in 216, Hannibal defeated the Romans again at the Battle of Cannae, where he annihilated the largest army the Romans had ever assembled. After the death or capture of more than 120,000 Roman troops in less than two years, many of Rome's Italian allies, notably Capua, defected to Carthage, giving Hannibal control over much of southern Italy. As Syracuse and Macedonia joined the Carthaginian side after Cannae, the conflict spread. Between 215 and 210 BC the Carthaginians attempted to capture Roman-held Sicily and Sardinia, but were unsuccessful. The Romans took drastic steps to raise new legions, enrolling slaves, criminals, and those who did not meet the usual property qualification and so vastly increasing the number of men they had under arms. For the next decade the war in southern Italy continued, with Roman armies slowly recapturing most of the Italian cities that had joined Carthage.