So I’ve been confused about something: I see Cannae constantly referred to as “double envelopment” but I can only see single envelopment in the battle (Hannibal encircling the Romans.) There’s only one cordon/ring of Carthaginians around the Roman army from the diagrams. Isn’t that single envelopment? Unless the “double” means it was coming from 2 sides at once?
It means pincer movement - envelopment along both flanks.
Right, if just the right flank, for example did a big loop around the Romans and met the stationary left flank, that would be a single envelopment.
Yep, Grey and ftg have it. Another famous double envelopment occurred at the Battle of Stalingrad; and the Germans had no better luck than the Roman did 2 millennia before.
On this topic, while I can understand in the modern army being cut off from supplies would mean that you’d have to break out if you wanted to eat eventually, my impression of the Battle of Cannae is that it was not a strategic encirclement but merely tactical. So the Persians had no direction to retreat to, so what? Why not just beat them in combat while encircled? Is it just that they panicked? If they had simply been better at fighting, would it have mattered that they were encircled? It seems like an encirclement doesn’t actually help you at all if you’re not winning on the soldier level to begin with, and merely allows you, when you do have the upper hand, to completely destroy the army you’re faced with instead of having them be able to run away and regroup later. It doesn’t seem like it actually provides the upper hand by itself, unless it’s just psychological.
[Bolding Mine] It was the Romans who were surrounded.
That is actually a good point, but we do have an answer. The short version is that the commander in charge, Gaius Terrentius Varro, reportedly packed his soldiers in as tight as possible on purpose, under the theory that they would completely overrun the Carthaginian position. This became a problem after the surrounding occurred - the soldiers simply couldn’t shift or re-deploy like a normal Roman army would. Second, once they began being beaten on either flank or the rear, there was nowhere for the Roman troops to escape. Only a handful were able to get away, including Varro himself.
My take is that the records suggest Varro was trying to put his men into something like a phalanx formation. Phalanxes were powerful in front but extremely vulnerable in the sides and rear, and the Roman equipment wasn’t really designed for that - although note that the Romans mostly used spears in battle at this stage, not primarily swords. Additionally, there were so many Roman soldiers present that they could easily have spared flanking and rear-guard forces, but Varro failed to assign any. His co-commander, Lucius Paulus, did not want to fight and specifically urged Varro not to fight on the open terrain where they could be easily flanked. Paulus died in the battle, after refusing the abandon his men.
A lot of it is psychological. Soldiers don’t fight well when there is an enemy to their rear, and when you are surrounded, it’s all rear.
IIRC Hannibal arranged his troops with the weak ones in the middle and the strong one at either side. This was an unusual thing to do. Usually the strong troops are in the middle.
The Romans had success pushing forward but as they did the sides of Hannibal’s army naturally enveloped the Romans. As the center was pushed back the sides slid down to the sides of the Roman army. The Romans, at first thinking they were doing well as the Carthaginian middle gave-way, did not realize their mistake till far too late. They literally walked in to it. The rest is, as they say, history. Supposedly it was so bad for the Romans that soldiers near the center kneeled down, dug a little hole in the ground with their hands and put their heads in to it to suffocate themselves rather than wait for the massacre they knew was coming.
Goes to show the value of a good general though.
This is correct, according to our sources. It probably would not have helped Hannibal had Varro not messed up his deployment. I don’t think we definitively know whether Hannibal did this before seeing Varro’s disposition, but it was probably after.
Something I didn’t mention was that the Romans were reportedly packed so tightly that some couldn’t even effectively fight. They simply were unable to raise their arms to strike.
So Hannibal called an audible? 
[sub](Sorry, football season just started)[/sub]
Hannibal planned this out at least a day or so ahead.
He put his strongest African troops on the flanks. He put the weaker Gaul-ally troops in the front center.
To shift troops around once you see how the other side is deployed would have been suicidal in those days. While you’re moving people around the other side attacks. End of battle.
Again the small area the core battle took place in is important as well as the fixed fighting positions a cohort would be set up in. Turning around and fighting in formation in place was just not doable. And if you weren’t fighting in formation you were dead meat.
The position of the centurion (or whatever they were called that early) was also bad for seeing what was going on and giving orders to the group.
One book I read about Hannibal mentioned that his cavalry would attack the rear along the line and slice the back of the legs of the Romans. The rear of these units was incredibly vulnerable.
Once the ordinary soldier in a pre-gunpowder battle saw which way the tide of battle was going against them, things just broke down. It becomes every man for himself all too easily.
Being encircled is a Very Bad Thing. It means you have lost the freedom of maneuver and you are basically at the enemy’s mercy. They can maneuver, pelt you with missiles, or just sit down and let you die of thirst if they feel like it. Even if the Romans hypothetically could have (A) recognized their situation, (B) communicated new instructions, and © organized a new defensive formation, they would have still been seriously disadvantaged. The defender’s only real options would be to attempt a breakout or hope relief arrived.
There ARE occasions when a defensive circle or square is desirable. The Duke of Wellington famously fought off repeated French cavalry charges at Waterloo by adopting defensive squares. Thing is, he was fighting a reverse slope defense and the French were not supported by infantry or artillery. It would not, therefore, be correct to say he was truly encircled.
As it was, the individual Roman likely had no idea what was happening until he started getting stabbed in the sides and rear. Once they realized they were surrounded they likely did begin to panic, and organizing a defense became that much more difficult.
We have a few definite cases where units were definitely re-arranged. If you knew the other guy wasn’t in an immediate position to attack you could get away with altering your disposition.
Hannibal undoubtedly had a battle plan in mind before, but he may not have arranged his soldiers like that until seeing how Varro messed up.
Thanks for the responses everyone.
A few more random thoughts/questions:
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It seems pretty incredible for a general in the pre-radio era to control formations like this. The din must be deafening and nobody could make their voice heard. Did Hannibal just tell his commanders, “We’re going to do a double envelopment,” and then have them pass it down word of mouth to the subordinates, and eventually down to the lowest levels, and everyone has to get on the same page somehow? Wouldn’t further orders be essentially impossible to give once the fighting began?
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Why can’t an encircled army “porcupine” itself and fight its way out if they heavily outnumber the encirclers (the Romans were almost 2:1 the Carthaginians?) In this case, the Romans were packed so tightly that many could not raise their arms to strike, but with a bit more room why couldn’t they break out by just hand-to-hand combat and let attrition (1 Roman dies, 1 Carthaginian dies, etc.) whittle down the Carthaginians?
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How well could an individual foot soldier grasp the overall situation? Seems they could only be aware of what’s happening within, say, an immediate 30-meter radius of them, due to the din and havoc.
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How do historians today know what happened so well, thousands of years ago, that they could make detailed graphs and diagrams of where specific units were and how they moved?
I’ll answer the last first, we don’t know. Nothing survives from the Carthaginian side. On the Romans, the best source is Polybius, who wrote 50 years later, but did have the chance to meet some of the survivors of the battle, including Scipio Not-Yet Africanus. Others include Livy and Appian, who write centuries later but had at least some access to primary sources, or is thought. So,while a lot is guessing it’s educated guesssing.
Secondly, lots of Romans did manage to fight their way out, like the aforementioned Scipio.
As for for the rest, the actual conduct of the battle is unclear. Adrian Goldsworthy suggests that what happened was that the Carthegian centre broke, probably in several places. Which happened the previous year at Lake Trasimene as well, but there hard fighting has managed to contain the breach, which was not supported. Hence Varro sent his last lines forward to exploit.
What all theorists agree is that as the Romans went forward in the center, they found the crack Libiyan infantry’s suddenly on their flanks. How that happened is still some matter of dispute. Hannibal placed a screen of skirmishes in front of the infantry. One conjecture is that the Romans therefore could not see the infantry on the flanks, or thought the skirmishes (who were dispatched pretty quickly) were it, and the bow shape of the line would have further masked them. So as they advanced forward so quickly, suddenly they were attacked by infantry on either flank, and the disposition of their attack formation was that they could not turn and face them quickly, while reserves who would have ordinarily taken on the task of driving the flankers away, were already engaged.
Of course the decisive shock came when the Carthaginian cavalry crashed into the rear of the Roman Army. This turned what was a defeat into a route, and the aforementioned reserves were not available or in formation to beat them back as they should have.
The cavalry should not discounted, a few years later at the Battle of Dertosa, Hasdrubal (Hannibal’s younger brother) failed to replicate Canne against the attacking Romans, despite the assaulting infantry being even more badly hit on the flanks, since the Roman cavalry kept the Carthagenians from attacking in the rear, at Cannae, the Cartehgenian cavalry once they drove the Italian horsemen off, attacked unmolested.
You need to bear in mind that communication during a battle was practically non-existent. A general would tell his commanders what he planned and maybe a few contingencies, and they would do the same with their own officers. Once the battle started there was no way to change the plan.
As to the why, lots of views.
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The traditional view, one which Polybius gave, is that Hannibal planned it to the last, making a virtue out of necessity, the inability of his Army to stand up to the Roman Legionaries in pitched battle, and employed his own advantage in cavalry, alongside.
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The second view is that Hannibal adjusted the battle plan as it went, of course the problem with that is he was in the centre, and once the Romans broke through, was essentially cut off from the other sectors, so controlling would have been difficult for him. (As an aside what the fuck is an “audible” ?)
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Another suggestion is that his actual plan was simply to get his cavalry to ride around the enemy Army and hit them in the rear, having pinned the center by drawing them in, he had no expectation the Romans would make such a meal of the flank battle; seriously they seem to have done the square root of fuck all once the Libiyans made contact. In that case, Cannae would have been a great victory, but not, well, Cannae.
Why the Romans fought so badly is much more difficult to answer. Varro clearly lost control, either by sending too many forward too soon, and not having reserves to deal,with a breakthrough or cavalry. For the former, let’s remember that Roman organisation was a lot looser than other contemporary armies and more flexible. Once battle was joined, it was the Centurions rather than the senior tribunes and legates who led the men, typically in smaller groups. This led to a greater adaptatibility and ability to exploit weaknesses and deal with setbacks. The epitome of this is seen at Pydna in 168 BC against the Phalanx of Macedon, the Romans had been driven back, but as the Macedones attacked, gaps appeared in their formations due to the broken ground, which Centurions in the middle took advantage of by entering the gaps and cutting down the pikemen (as an aside, Varro was certainly not trying to get the Romans into a Phalanx, the Romans had abandoned that nearly a century ago as being too rigid). At Cannae, it seem that this tactical and organisational flexibility hurt the Romans, as the centurions saw the guys in front going forward, they themselves did, the guys behind them saw this and took their guys forward and so on. The latter is a lot more difficult to explain, he should have known that a Cavalry defeat was likely, and he should have planned to leave reserves to deal with that. If he had done so, would have been a different battle.
“Audible” - to change the play at the line of scrimmage (American football). Normally the quarterback will call the next play in the huddle. When they line up at scrimmage, however, he may decide that the defensive formation is wrong for the play he just called. Either he spotted a weakness that could be exploited or he saw the potential for getting his ass handed to him. In which case he will loudly call out a new play (an “audible.”)
It’s another term for “pulling a new plan out of your ass on the fly.” 
An audible is when a football quarterback sees something in the opposing defense that makes it better to go with a different play instead, and so he calls out the new play for the offense to use.
Edit: Silenus beat me to it.