The 2nd of August is the anniversary of the Battle of Cannae (216 BC), one of Rome's great defeats - and which led to the accusation of incestum against a Vestal Virgin (something must have angered to gods for Rome to lose so badly, so someone must have done something wrong ...).
Lucius Aemilius Paullus and Gaius Terentius Varro had almost twice the number of men Hannibal had in Apulia (sixteen legions to his ca. 50,000), but the Carthaginian was tactically superior. The simplest way to describe what he did is to say that he drew them into an arc, encircled them, then slaughtered them - up to 70,000 Romans died, including the consul Aemilius, the two consuls from the previous year, and 80 of the 300 Senators; possibly another 4,000 were captured.
Cannae is the battle that makes Hannibal a great, and which all successive generals hope to emulate - annihilation of the opponent.
Rome lost 85% of her army, up to 75,000 men, and much of her ruling class at Cannae. This would stand as a record until the 6th October 105 BC when 80,000 Roman soldiers and some 20,000 followers would be massacred by the Cimbri and Teutones at the Battle of Arausio. The combination of the fear of the 'Gauls,' and his ability to defeat them following this great loss, was what made Gaius Marius so great a general.
After Cannae Rome's Allies began to desert her, and the whole of southern Italy was open to Hannibal. The obvious course would have been to march on Rome, though Hannibal realised that this would have stretched her resources. By the rules of ancient warfare the Romans should have capitulated after such a great defeat, and become part of the Punic Empire - that they did not is one of the things that made them great, and ultimately led them crush Carthage.
The defeat had shown the Romans that they needed to change the way they fought - if they were going to ignore the rules about when to submit, they might as well throw out the rules of Greek warfare too. The phalanx was abandoned in favor of columns, and greater flexibility (Marius further broke down the structure into cohorts). The command structure simplified - one consul was in charge rather than both. The changes were quickly implemented, so that after the Battle of Zama (202 BC), Scipio Africanus was able to bring the Second Punic War to a close (201 BC).