Battle of Cannae
No other nation surely would not have been overwhelmed by such an accumulation of misfortune.
For a brief period, the Romans were in complete disarray. Their best armies in the peninsula had been destroyed, the few remnants severely demoralized, and the only remaining consul (Varro) completely discredited. As the story goes, Rome declared a national day of mourning as there was not a single person who was not either related to or acquainted with a person who had died. The Romans became so desperate that they resorted to human sacrifice, twice burying people alive at the Forum of Rome and abandoning an oversized baby in the Adriatic Sea (perhaps one of the last instances of human sacrifices by the Romans, apart from public executions of defeated enemies dedicated to Mars).

Within just three campaign seasons (20 months), Rome had lost one-fifth (150,000) of the entire population of male citizens over 17 years of age. The morale effect of this victory was such that most of southern Italy joined Hannibal's cause. After Cannae, the Hellenistic southern provinces of Arpi, Salapia, and Uzentum, including the cities of Capua and Tarentum, two of the largest city-states in Italy, and other settlements of non-Latin origin such as  Herdonia, revoked their allegiance to Rome and pledged their loyalty to Hannibal. 
As Livy noted, "How much more serious was the defeat of Cannae than those which preceded it, can be seen by the behavior of Rome's allies; before that fateful day, their loyalty remained unshaken, now it began to waver for the simple reason that they despaired of Roman power." Following the battle, Sicily's Greek cities rose in revolt against Roman political control. The Macedonian king, Philip V, pledged his support to Hannibal, initiating the First Macedonian War against Rome. Hannibal also secured an alliance with the new king Hieronymus of Syracuse, the only independent king left in Sicily.

Livy illustrates the state of Roman morale with two vivid anecdotes. The first concerns Hannibal's brother Mago, who had returned to Carthage with news of the victory. He reported to their senate that in several engagements with the Romans Hannibal had killed over 200,000 soldiers and taken 50,000 prisoner; of six commanders, two consuls and a Master of horse had been slain; and a number of Roman allies had gone over to the Carthaginians. Then Mago concluded his report by having a collection of golden rings poured upon the council floor in front of the assembled senators. He explained that each ring belonged to one eques who had been slain in battle and had earned the ring through exceptional bravery. Livy notes that one unnamed authority stated the volume of jewelry amounted to three and one-half measures (Congius?), only to add "it is generally and more credibly held that there was not more than one measure of them".
The second concerns Lucius Caecilius Metellus and three other military tribunes, who had taken refuge at Canusium with other Roman refugees. Demoralized at the defeat, they discussed the possibility of setting sail overseas and finding employment as mercenaries for some foreign prince. Word of this meeting reached the young Publius Cornelius Scipio who, with only a few followers, burst into the room where the discussion was underway. Holding his naked sword over the heads of the wavering men, Scipio is reported to have cried:

I swear with all the passion in my heart that I will never desert our homeland, or permit any other citizen of Rome to leave her in the lurch. If I willfully break my oath may Jupiter, Greatest and Best, bring me to a shameful death, with my house, my family, and all I possess! Swear the same oath, Caecilius! And the rest of you, swear it too. If anyone refuses, against him this sword is drawn.
After the battle, the commander of the Numidian cavalry, Maharbal, urged Hannibal to seize the opportunity and march immediately on Rome. It is told that the latter's refusal caused Maharbal's exclamation: "Assuredly, no one man has been blessed with all God's gifts. You, Hannibal, know how to gain a victory; you do not know how to use it."
Instead, Hannibal sent a delegation led by Carthalo to negotiate a peace treaty with the Senate on moderate terms. Despite the multiple catastrophes Rome had suffered, the Senate refused to parley. Instead, they redoubled their efforts, declaring full mobilization of the male Roman population, and raised new legions, enlisting landless peasants and even slaves. So firm were these measures that the word "peace" was prohibited, mourning was limited to only 30 days, and public tears were prohibited even to women.: 386 
Hannibal had good reasons to judge the strategic situation after the battle differently from Maharbal. As the historian Hans Delbrück pointed out, due to the high numbers of killed and wounded among its ranks, the Punic army was not in a condition to mount a direct assault on Rome. It would have been a fruitless demonstration that would have nullified the psychological effect of Cannae on the Roman allies. Even if his army were at full strength, a successful siege of Rome would have required Hannibal to subdue a considerable part of the hinterland to cut the enemy's supplies and secure his own.
Even after the tremendous losses suffered at Cannae and the defection of a number of her allies, Rome still had abundant manpower to prevent this and maintain considerable forces in Iberia, Sicily, Sardinia and elsewhere, despite Hannibal's presence in Italy. Hannibal's conduct after the victories at Trasimene (217 BC) and Cannae, and the fact that he first attacked Rome only five years later in 211 BC, suggests that his strategic aim was not the destruction of his foe but to dishearten the Romans by carnage on the battlefield and to wear them down to a moderate peace agreement by stripping them of their allies.
"In fact there were many good reasons for not marching on Rome," military expert Robert O'Connell writes, "and only one good reason for going." While the scholars provide reasons not to march, a soldier, Field Marshal Bernard Montgomery, believed Maharbal was right; when a more powerful adversary is down, he must be dispatched. "Hannibal's single chance of winning the larger war was to begin marching his army towards Rome," O'Connell adds. "In the end it still would have been a long shot. But it was his only shot. Instead, Hannibal chose another route, and the war became only a matter of time."
For the remainder of the war in Italy, the Romans did not amass large forces under one command against Hannibal; they used several independent armies, still outnumbering the Punic forces in numbers of soldiers. The war in Italy still had occasional battles, but was focused on taking strongpoints and constant fighting according to the Fabian strategy. This finally forced Hannibal with his shortage of manpower to retreat to Croton from where he was called to Africa for the final battle of Zama, where the Roman victory ended the war.

Historical significance: 

Effects on Roman military doctrine: Cannae played a major role in shaping the military structure and tactical organization of the Roman Republican army. At Cannae, the Roman infantry assumed a formation similar to the Greek phalanx. This left them vulnerable to Hannibal's tactic of double envelopment since their inability to maneuver independently from the mass of the army made it impossible for them to counter the strategic encirclement used by the Carthaginian cavalry. The laws of the Roman state requiring command to alternate between the two consuls restricted strategic consistency.
In the years following Cannae, striking reforms were introduced to address these deficiencies. First, the Romans "articulated the phalanx, then divided it into columns, and finally split it up into a great number of small tactical bodies that were capable, now of closing together in a compact impenetrable union, now of changing the pattern with consummate flexibility, of separating one from the other and turning in this or that direction." For instance, at Ilipa and Zama, the principes were formed up well to the rear of the hastati—a deployment that allowed a greater degree of mobility and maneuverability. The culminating result of this change marked the transition from the traditional manipular system to the cohort under Gaius Marius, as the basic infantry unit of the Roman army.
In addition, a unified command came to be seen as a necessity. After various political experiments, Scipio Africanus was made general-in-chief of the Roman armies in Africa, and was assured this role for the duration of the war. This appointment may have violated the constitutional laws of the Roman Republic but, as Delbrück wrote, it "effected an internal transformation that increased her military potentiality enormously" while foreshadowing the decline of the Republic's political institutions. Furthermore, the battle exposed the limits of a citizen-militia army. Following Cannae, the Roman army gradually developed into a professional force.

Status in military history: Cannae is as famous for Hannibal's tactics as it is for the role it played in Roman history. Not only did Hannibal inflict a defeat on the Roman Republic in a manner unrepeated for over a century until the lesser-known Battle of Arausio, but the battle also has acquired a significant reputation in military history.