Battle of Cannae
Hannibal's cavalry also came from diverse backgrounds. He commanded 4,000 Numidian, 2,000 Iberian peninsular, 4,000 Gallic and 450 Libyan-Phoenician cavalry. Hannibal had an auxiliary skirmisher contingent consisting of 1,000–2,000 Balearic slingers and 6,000 mixed-nationality javelinmen, possibly including Lusitanians among them. The uniting factor for the Carthaginian army was the personal tie each group had with Hannibal.

Equipment: Rome's forces used typical Roman equipment including pila (heavy javelins) and hastae (thrusting spears) as weapons as well as traditional bronze helmets, bodyshields and body armor. In contrast, the Carthaginian army used a variety of equipment. The Iberians fought with falcatas, while Celtiberians and Lusitanians used straight gladii, as well as javelins and various types of spears. For defense, warriors from Hispania carried large oval shields and often wore a crested helmet made of animal sinews. Most Gallic foot warriors likely had no protection other than large shields, and the typical Gallic weapon was a long slashing sword. 
The Numidian cavalry were very lightly equipped, lacking saddles and bridles for their horses, and wearing no armor but carrying small shields, javelins and possibly a knife or a longer blade. In contrast, the heavier Iberian peninsular cavalry carried round shields, swords, javelins and thrusting spears. The similarly heavy Gallic cavalry added the four-horned saddle, with the wealthier ones being clad in mail, a Gallic invention. 
Skirmishers acting as light infantry carried either slings or javelins. The Balearic slingers, who were famous for their accuracy, carried short, medium and long slings used to cast stones or bullets. They may have carried a small shield or simple leather pelt on their arms, but this is uncertain. Hannibal himself, like many Roman officers on the opposing side, might have been wearing a bronze musculata and carrying a falcata as his personal sidearm.
The equipment of the Libyan line infantry has been much debated. Duncan Head has argued in favor of short stabbing spears. Polybius states that the Libyans fought with equipment taken from previously defeated Romans. It is unclear whether he meant only shields and armor or offensive weapons as well, though a general reading suggests he meant the whole panoply of arms and armor, and even tactical organization. Apart from his description of the battle itself, when later discussing the subject of the Roman legion versus the Greek phalanx, Polybius says that "...against Hannibal, the defeats they suffered had nothing to do with weapons or formations" because "Hannibal himself... discarded the equipment with which he had started out [and] armed his troops with Roman weapons". 
Gregory Daly is inclined to the view that Libyan infantry would have copied the Iberian use of the sword during their fighting there and so were armed similarly to the Romans. Peter Connolly has argued that they were armed as a pike phalanx. This has been disputed by Head, because Plutarch states they carried spears shorter than the Roman triarii and by Daly because they could not have carried an unwieldy pike at the same time as a heavy Roman-style shield.

Tactical deployment: The conventional deployment for armies of the time was placement of infantry in the center, with the cavalry in two flanking wings. The Romans followed this convention fairly closely, but chose extra depth rather than breadth for the infantry in hopes of breaking quickly through the center of Hannibal's line. Varro knew how the Roman infantry had managed to penetrate Hannibal's center at Trebia, and he planned to recreate this on an even greater scale. The principes were stationed immediately behind the hastati, ready to push forward at first contact to ensure the Romans presented a unified front. As Polybius wrote, "the maniples were nearer each other, or the intervals were decreased... and the maniples showed more depth than front". 
Even though they outnumbered the Carthaginians, this depth-oriented deployment meant that the Roman lines had a front of roughly equal size to their numerically inferior opponents. The typical style of ancient warfare was to continuously pour infantry into the center and attempt to overpower the enemy. Hannibal understood that the Romans fought their battles like this, and he took his outnumbered army and strategically placed them around the enemy to win a tactical victory.

Hannibal had deployed his forces based on the particular fighting qualities of each unit, taking into consideration both their strengths and weaknesses. This aspect of Hannibal's leadership was highlighted in the use of a Spanish unit, the Balearic slingers, whom he placed behind the infantry to hurl their ranged missiles into the masses of Roman troops. He placed his Iberians, Celtiberians and Gauls in the middle, alternating the ethnic composition between Hispanics and Gauls across the front line, with himself at the front and center alongside his brother Mago. 
Roman sources claim their placement was chosen for being the most expendable and unreliable troops, but modern reflections believe those forces were actually selected for their battle-hardening to carry the weight of the Punic side, as they would be tasked with the controlled retreat that ultimately made possible Hannibal's pincer movement. Meanwhile, infantry from Punic Africa was on the wings at the very edge of his infantry line. This infantry would remain cohesive and attack the Roman flanks.
Hasdrubal led the Hispanic and Gallic cavalry on the left (south near the river Aufidus) of the Carthaginian army. By placing the flank of his army on the Aufidus, Hannibal prevented this flank from being overlapped by the more numerous Romans. Hasdrubal was given 6,000–7,000 cavalry, and Hanno had 3,000–4,000 Numidians on the right.
Hannibal intended that his cavalry, comprising mainly medium Hispanic cavalry and Numidian light horse, and positioned on the flanks, would defeat the weaker Roman cavalry and swing around to attack the Roman infantry from the rear as it pressed upon Hannibal's weakened center. His veteran African troops would then press in from the flanks at the crucial moment, and encircle the overextended Romans.
The Romans were in front of the hill leading to Cannae and hemmed in on their right flank by the river Aufidus, so that their left flank was the only viable means of retreat. In addition, the Carthaginian forces had maneuvered so that the Romans would face east. Not only would the morning sun shine low into the Romans' eyes, but the southeasterly winds would blow sand and dust into their faces as they approached the battlefield. Hannibal's deployment of his army, based on his perception of the terrain and understanding of the capabilities of his troops, proved decisive.

Battle: As the armies advanced on one another, Hannibal gradually extended the center of his line, as Polybius described: "After thus drawing up his whole army in a straight line, he took the central companies of Hispanics and Celts and advanced with them, keeping the rest of them in contact with these companies, but gradually falling off, so as to produce a crescent-shaped formation, the line of the flanking companies growing thinner as it was prolonged, his object being to employ the Africans as a reserve force and to begin the action with the Hispanics and Celts." Polybius described the weak Carthaginian center as deployed in a crescent, curving out toward the Romans in the middle with the African troops on their flanks in echelon formation. 
It is believed that the purpose of this formation was to break the forward momentum of the Roman infantry, and delay its advance before other developments allowed Hannibal to deploy his African infantry most effectively. While the majority of historians feel that Hannibal's action was deliberate, some have called this account fanciful, and claim that the actions of the day represent either the natural curvature that occurs when a broad front of infantry marches forward, or the bending back of the Carthaginian center from the shock action of meeting the heavily massed Roman center.
The battle began with a fierce cavalry engagement on the flanks. Polybius described many of the Hispanic and Celtic horsemen facing the Romans dismounting due to the lack of space to fight on horseback, and called the struggle "barbaric" in the sense of its utter brutality. When the Carthaginian cavalry got the upper hand, they cut down their Roman opponents without giving quarter. On the other flank the Numidians engaged in a way that merely kept the Roman allied cavalry occupied. Hasdrubal kept his victorious Hispanic and Gallic cavalry under control and did not chase the retreating Roman right wing. Instead, he led them to the other side of the field to attack the socii cavalry still fighting the Numidians. Assailed from both sides, the allied cavalry broke before Hasdrubal could charge into contact and the Numidians pursued them off the field.
While the Carthaginian cavalry were in the process of defeating the Roman horsemen, the masses of infantry on both sides advanced towards each other in the center of the field. The wind from the east blew dust in the Romans' faces and obscured their vision. While the wind was not a major factor, the dust that both armies created would have been potentially debilitating to sight.