Dwight Eisenhower once wrote “every ground commander seeks the battle of annihilation; so far as conditions permit, he tries to duplicate in modern war the classic example of Cannae," holding Hannibal’s victory over Rome as the gold standard of tactical victories.
Almost everyone with even a cursory interest in military history knows that this battle is the classic example of the “double envelopment”—the surrounding of an enemy army on both flanks, cutting off the enemy’s escape to annihilate them.
What gets lost sometimes in this is understanding how Hannibal actually achieved this maneuver.
Double envelopments are quite rare in military history. I would generally categorize them into a few recognizable patterns, based on how the victorious general achieved the maneuver.
- Quality of Troops
- Quantity of Troops
- Mobility of Troops
- Ambush
An example of quality of troops leading to double envelopment is the Battle of Marathon (490BC). A smaller Greek Army faced off with a much larger Persian army bracketed by a river and a marsh. The Greek Army used the superiority of their bronze armored heavy infantry to destroy the Persian flanks before sweeping towards the center to achieve a double envelopment.
Using quantity of soldiers to achieve a double envelopment is even easier to understand, but not overly common. Rarely are generals willing to let a larger army simply surround them. By having more soldiers move aggressively to overlap the enemy flanks, an aggressive general can surround the enemy, and this is achieved operationally in some cases. Much like how Napoleon’s 80,000 troops caught the Austrian General Mack’s 40,000 unaware to surround them pinned against a river before a single shot was fired at the Battle of Ulm.
Even a numerically smaller army can achieve a double envelopment against an enemy if they have superior mobility. The Mongols frequently made use of the superior speed of their lightly armored horse mounted archers, each with 5 or more horses that they can switch to, when fighting infantry or heavily armored and slow cavalry—such as at the Battle of Kalka River.
Finally, ambushes often take the form of encirclements. A favorite tactic of the Shimazu Clan during the Warring States period in Japan, at the Battle of Hetsugigawa (1587) a feigned retreat by a forward unit drew the numerically superior enemy army into a valley surrounded on 3 sides by hidden Shimazu forces, whereupon the enemy army was encircled and destroyed.
In each of these cases, a very simple “advantage” exists for the army conducting the encirclement—quality, quantity, mobility or surprise.
One thing that makes Cannae different is that Hannibal held none of these advantages at this battle.
The Armies
The Second Punic War is the second major conflict between Carthage (based in modern day Tunisia) and Rome.
Seeking to avenge their defeat in the First Punic War, Hannibal Barca and the Barca family had established a new power base by conquering what’s now southern Spain and its silver mines. Hannibal used this money to bring together an army, but it was of greatly varying quality.
Hannibal had 40,000 infantry and 10,000 cavalry at the battle.
Hannibal’s infantry was of varying quality. Almost half his infantry (19,000) were Gallic infantry, from modern day Southern France, but they were lightly armored and considered of poor quality. Hannibal did have 8,000 Iberians and 8,000 Libyans, who were more heavily armored and considered good infantry.
By contrast, the Romans brought 80,000 infantry and 6400 cavalry to the battle, giving them twice as much infantry as the Carthaginians. Furthermore, Rome and its allies were fairly uniformly equipped and trained, and were considered of generally high quality.
Hannibal’s cavalry was considered one of his few advantages as they were of superior quality and number, but Hannibal would complete his double envelopment without needing the help of his cavalry—thus effectively, Hannibal encircled an army double his size with inferior troops without any advantage in mobility or surprise.
The question is: how?
Hannibal’s Tactical Genius
Traditionally, the blame on the Roman side is given to one of two Roman Consuls in command at the battle: Gaius Terentius Varro. Polybius (the primary ancient source on the battle) places Varro as in command on the day of the battle, and virtually entirely absolves the other Consul, Lucius Aemilius Paullus of any blame for the defeat.
Some historians point out that Polybius’ patron was Scipio Aemilianus, who also happens to Paullus’ grandson, thus question whether Varro (who also lacked any prominent descendants who could defend him) had been made a scapegoat, but lacking other sources it is difficult to determine.
Conventional deployment called for placing cavalry at the flanks, infantry in the center. Furthermore, it was common for the best infantry to be placed in the center, in front of the commanding general.
The Romans followed convention in their deployment, with the only tweak being that they chose to concentrate the depth of their formation at the center of their formation. The Romans had previously penetrated Hannibal’s center during the Battle of Trebia (albeit in a defeat), and the Romans may have seen Hannibal’s center as a weakness to exploit.
Hannibal likewise deployed his soldiers with cavalry on the flanks and infantry in a much thinner line, matching the length of the Roman formation with half the troops. But going against convention, Hannibal placed his best soldiers (the Libyans and Iberians) on the flanks, and placed his weakest soldiers (the Gauls) in his center.
Furthermore, instead of advancing by line abreast, Hannibal advanced on the Romans in a wedge formation, much like the letter “V” pointed at the center of the Roman line. It’s here where Hannibal’s psychological trap was laid.
Normally, when an army forms a wedge, the intent is for the Wedge to pierce the enemy line. The point of the wedge will have the best troops, who run into the enemy line first. The idea is that the elite troops will punch through the enemy line, thus when your inferior troops follow up on the flanks, the enemy formation has already lost cohesion.
It is an excellent way for a smaller but elite force, led by exception soldiers in the vanguard to defeat larger armies of inferior quality.
For example, in generations past, Alexander the Great’s Macedonian heavy cavalry used wedge formations to pierce Persian defenses, a fact which both Romans and Carthaginians would likely have been aware.
Thus, it may have appeared to the Romans that Hannibal intended to pierce the Roman line at their center. Yet Hannibal had placed his weakest Gallic troops at his center. This can hardly seem accidental.
Obviously, the Gallic soldiers came into contact with the Roman center before anyone else. They would have had difficulty punching through an equal number of Roman heavy infantry, but to do so when the Romans concentrated their forces at their center was hopeless.
instead, without losing unit cohesion, the Gauls began slowly retreating without losing contact with the Roman center. Fighting sporadically, they would retreat a few yards, fight, then retreat again. This type of slow withdrawal minimized any losses they suffered, while drawing the Romans forward.
What happens next is not disputed—the Roman Army compacts itself towards the center of the formation and pushes forwards. The powerful two flanks of the Carthaginian force stands firm while the center retreats, until the center retreats past the flanks. So the Carthaginian army transitions from a forward facing “V” formation, to a backward facing “U” formation.
By that time, the entire Roman Army settles inside the Carthaginian formation, completing Hannibal’s double envelopment.
Polybius attributes this maneuver primarily to Varro’s hubris and stupidity. Thinking that the Carthaginians were in flight, Varro supposedly orders the entire Roman army to charge the Carthaginian center, falling into Hannibal’s trap and dooming the Roman army (with no blame for Paullus).
However, some modern historians question whether Varro had any practical control particularly over the wide ranging flanks of this army in the cacophony and confusion of a major ancient battle, particularly of this era of Ancient Combat.
An alternative explanation, looks to the human instinct to “attack fleeing enemies.”
As the initial unit of Gauls attacks, then is pushed back, soldiers on either side of the unit are not fighting anyone, and instinctively move to pursue the closest (fleeing) soldiers.
As subsequent units get pulled in and also join the retreat, further and further down the Roman line, troops begin pressing not directly forwards, but diagonally slanted towards the center.
With the Roman units already aligned facing towards the center, it’s easier for the flanking units on edges of the Carthaginian formation to force the flanks of the Romans inward, to complete the double envelopment.
Roman defeat became total when the Carthaginian cavalry, who had been fighting a more conventional cavalry on cavalry fight on the flanks, defeated their Roman counterparts and attacked the rear of the Roman formation.
Ancient sources vary on the losses reported, but between 70% to 85% of the Roman army is believed to have been killed or captured, an annihilation of the main Roman Army.
Even when an army attacks the center of the enemy formation and fails, it is quite rare for (particularly a numerically superior army) to become double enveloped in this manner. Thus it is difficult to buy the idea that pure Roman stupidity or luck could result in such catastrophic consequences.
Thus it becomes necessary to look to Hannibal’s tactics, and the unconventional use of the wedge formation, which reveals the deft pyschological trap that Hannibal laid to invite the Roman army to double envelop itself.
How might the Romans have avoided such a defeat?
Even a relatively ordinary general of the Late Roman Republic might have had little trouble avoiding Hannibal’s trap as it was laid here.
One major innovation of the late Republic was the regular use of reserves.
Rather than placing all 80,000 infantry on the front line, in the later republic it would have been conventional to keep a tactical reserve, perhaps 10-20,000 soldiers held back in reserve units.
Even if the front line were to be sucked into Hannibal’s trap, the Roman general would have his reserve legions to commit once he saw what was developing, strengthening his flanks, or perhaps even crushing Hannibal’s flanks.
The greater flexibility and tactical prowess of late Republican and Imperial Roman Armies can be observed from the offensive and defensive uses of reserves. And it’s quite apparent that Late Roman Republican Armies could use that flexibility to make themselves far less prone to unexpected tactics.