The Makedonian phalanx -- why such depth?

[PMBardunias]

I have not come across a case of Mass Casualties occurring in the open, without 'immovable objects', or for that matter in instances of crowd pushing against crowd, as in a riot for example.I am skeptical that such events happen.

With respect Paul, as I have mentioned before it is irrelevent to the physics as to what the mass is pushing against. Anything that can push back with enough force to resist the advance of the crowd will do. This can be a retaining barrier, but it can also be other people in a crowd. Even when fatalities occur in a crowd of the type you are thinking of, it is not simply because people are crushed up against a wall, but because they are crushed between people resisting being crushed up against a wall and people behind pushing them forward. The "wall" is irrelevent to all of the people pushing forward behind those immediately up against the wall. The wall simply creates a condition where a bottle-neck occurs. No crowd of rational people want to be in a lethal density, thus if they were free to move they would.

Riots are different all together for they are a single crowd being "herded" by police through physical and psychological threat. They could achieve crowd-like density if encircled as those on the outside backed up into the group to avoid the threat from police. This is essentially what happened at Cannae for example.

There are examples of crowds colliding along streets for example or famously in a very lethal incident in Mexico where part of the crowd was trying to get back into a football stadium while the other part was trying to get out. So yes, there are examples of crowds converging lethally, but there must always be some constraint to movement that causes a crowd to form dense or people would just move past each other. Hoplites are obviously not going to let their foes simply pass through their ranks, but will resist them, by pushing if need be.

In the context of phalanx combat we have two groups of men who are actively resisting each other, not acting like rational pedestrians. Thus they themselves form equal and opposite forces which lead to a each side forming as if a crowd against a wall (the wall being the other phalanx, which is impenitrable, a literal shield "wall" in this sense). They were free to disengage at any time- what we would call a rout- and frequently did.

Asking if there is an example of two groups of people in an open field who crashed into each other like lethal crowds is a bit like asking for an example of pedestrians bumping into each other on the street and breaking out into a full-blown Sumo pushing match. They surely could do it, but why would they? Put the same two guys trying to push through a narrow door in a burning building and you will see lots of pushing.

So why would hoplites do it? Well the short answer is "because they can". More precisely the reason is that nothing short of a crowd (or a wall and it better be real strong because brick walls have been knocked down) can stop a crowd's advance. The obvious way to destroy the cohesion of a linear formation is to disrupt its linearity by pushing through it- which is much faster and safer than trying to kill your way through it. Crowd densities naturally emerge as rear ranks move up to support those in front, the process is gradual and smooth so there is never a moment where the ranks in front are being knocked forward. Unpacking is just as gradual so there is no moment when front rankers suddenly have no resistance and fall forward. They are free to pack and unpack just as cars do on a highway during a traffic jam. You will notice that you (usually) do not plow into the car in front of you as it comes to a stop.

Of course there is an element of "game" involved here because you could simply not resist the crowd and with each few steps back they cannot achieve crowd density. But this required a major shift in tactics, essentially becoming lighter infantry (or perhaps Iphicratids or thureophoroi). What occurred instead early on was an "arms race" where phalanxes got deeper and probably better coordinated. We also see the exploitation of the other main weakness of a linear formation, flanking attacks. But perhaps the best way to short circuit a hoplite phalanx was to force them to push suicidally against a hedge of sharp-pointed sarissa. Othismos and its relative importance as a phase of phalanx combat explains much about the evolution of Greek warfare and the form and function of the panoply.