What do men in the middle of a phalanx contribute?
says that phalanx are vulnerable from behind
So what does people in phalanx formation when they get smacked from behind? They just keep pointing their spears to the front?
Why don't they just aim their spears to the back?
Now, in 5 row phalanx this may be an issue. But in 10 rows I do not see this as an issue.
And what's with the left of phalanx unprotected? They don't have lefties holding shield on right arms?
I think the key here is to appreciate just how long the front of a phalanx was. A phalanx really only worked in one continuous line. Each man at the front held anywhere from half a meter (synaspismos, shields locked) to one meter (pycne, normal battle formation). 8000 hoplites arrayed 8 deep means a front of 500 to 1000 meters! This was done, in part, because it's extremely difficult for an ancient commander to see what's going on or to issue orders. One, big continuous front was relatively easy to train for and maintain.
In contrast, the flanks are a mere 8 men, 8 meters long. They could provide no significant defense. The enemy would simply flow around to the rear. To prevent this, the flanks of the army would either extend, or they would retreat inward forming a bowed shape to protect the flanks and rear.
The rear ranks could, and probably did, turn to face a threat from the rear. This is why phalanxes weren't regularly defeated by lightly armed skirmishers and cavalry simply racing around to the rear. But if heavier units got behind them, this meant they were now fighting surrounded. Nobody likes to fight surrounded and this would have a crushing effect on morale. This was not how they were trained to fight, the main weakness of a phalanx was its inflexibility, and the infantry square had not yet come into fashion. Being attacked from a new direction also risked damaging the cohesion of the unit as individual soldiers were now being pushed and attacked from multiple directions.
Publius Flavius Vegetius Renatus stated in De re militari
No part of drill is more essential in action than for soldiers to keep their ranks with the greatest exactness, without opening or closing too much. Troops too much crowded can never fight as they ought, and only embarrass one another. If their order is too open and loose, they give the enemy an opportunity of penetrating. Whenever this happens and they are attacked in the rear, universal disorder and confusion are inevitable.
Ancient battles weren't particularly lethal, instead it was about breaking their tight, defensive formations. When you see in the movies two armies run at each other and the fight immediately devolves into individual combat, that's movie nonsense. Ancient armies would fight for hours in tight formations until exhaustion, casualties, or clever tactics broke them, or nightfall simply ended the battle.
One of the main improvements of the Macedonian phalanx was the employment of significant amounts of light and heavy cavalry to protect their flanks and harass the enemy. In addition, the men making up the phalanx were professional soldiers and could be drilled in more complex maneuvers. The Macedonian phalanx would typically pin the enemy in place while the light cavalry dealt with skirmishers and the heavy cavalry attacked the enemy's flank.
The Romans improved on this with the maniple as their basic unit, employed to great effect against the Samnites around 300 BC. 120 men where formed into a front 40 men wide and 3 men deep. This smaller, well trained unit allowed for much greater tactical flexibility. Even so, attacks from the rear invited chaos.
The Romans also introduced a circle formation into training to fight when surrounded. Again, De re militari...
They must be taught to form the circle or orb; for well-disciplined troops, after being broken by the enemy, have thrown themselves into this position and have thereby prevented the total rout of the army. These evolutions, often practiced in the field of exercise, will be found easy in execution on actual service.
This evolved into the infantry square employed by the Romans at the Battle of Carrhae 53 BC. At Carrhae each side was 12 cohorts or about 7,200 men per side! While this succeeded in giving them all around protection, it limited their mobility and they were harassed by horse archers throughout the battle.
The infantry square would continue to be infantry's response to being surrounded or cavalry attack all the way up to the Battle of Waterloo until rapid fire rifles made them obsolete.
I regularly take part in big medieval fighting events, so I speak from experience rather than citing sources:
When you are attacked from two sides, your troops get pushed together until they are unable to move and strike/block effectively. They are standing on each others toe, so to say. The large battles are unlike any personal fight or brawl. A lot is decided by initiative, morale and pure physical pressure rather than individual injuries.
Another aspect is situational awareness. In a battle-situation you are at your best if you focus right on your enemy. Having to watch your back at the same time diverts attention. It may even go so far that you get tunnel-vision and do not realize a surprise-attack to your back until it´s to late.
Several reasons why your idea won't - and cannot - work. First of all, you have to have a very, very bad general if he allows his phalanx(es) being attacked in the rear. So bad, that if he survives the battle, he'll be very likely murdered by his surviving troops or executed by his government. People will wonder how much the enemy paid him for his services.
A phalanx is a (a kind of) unit. An army usually had several of them. Also skirmish units, archers and cavalry and other units. The job of those troops is to make sure that attacks from the rear don't happen.
You need a very good general to get out of that mousetrap. Caesar got himself into that predicament in Thapsus. He was surrounded on a peninsular on both ends. He got out of it, but made damn sure that never ever happened again under his command.
Next, as has already been stated - turning around will not work. Everybody got less room to fight. You're squeezed into not being able to do anything. Not even wield a sword to defend yourself. This was essentially what Hannibal did to the Romans at Trasimene and Cannea.
Phalanxes weren't the only formation with that particular problem. Almost all formations have their pro's and cons. The Romans didn't use phalanxes at Trasimene, Cannea and Thapsus, but they were squeezed in nevertheless.
You also wonder why nobody wore a shield on the left side. I am left handed, but when I started epee fencing, I noticed I am right handed for that. It's usually a mistake to force people to do the other thing. Left handed writers can learn to write right handed, but usually have a lousy handwriting. And vice versa. I can do calligraphy with my left hand, and learned right hand writing - which is barely readable. About the level of a first grader.
Same applies to fighting. You can train yourself to fight right handed, but you'll never be really good at it. The soldiers no doubt passed the requirements, otherwise they wouldn't be in a phalanx or a legion. I doubt if they were much better than the basic requirements.
Next, not that many people are left handed by nature. About 10%, I read. That's not a lot to start working with. The other 90% would be equally bad when trained left handed. So the obvious and better solution was to have a different unit cover the left flank. That's where a.o. cavalry and light infantry comes into play.
