(I realize this may be a way-too-general question, so if it helps I'm specifically interested in forms of unit-based warfare prior to the invention of massed fire; medieval European/Sengoku Japanese/Greek hoplite-type stuff.)
I've always been interested in the logistical qualities of how an actual battle is run, what with all the screaming and the dying and the chaos of trying to get large groups of people to do anything with any sort of cohesion. But one thing that I've been wondering for a while is: standing in the front line and charging another unit, especially one that's had time to place their spears, essentially seems like asking a bunch of folks to run full-speed at a wall of sentient knives. I get that mid-way through the charge the mass of bodies probably made individual qualms moot in the face of inertia, but how did soldiers overcome the terror of what seems like fairly certain death? For that matter, was it as impossibly deadly as it sounds to try and avoid being immediately impaled? How were these soldiers selected? (The most-skilled would seem to make sense to have the best chance of surviving, but also this seems like a great way to sacrifice your best folks early on.) And finally, were there special provisions set up for unit leaders, bannermen, musicians, commanders, etc?
I realize that head-to-head unit-to-unit is probably the least ideal matchup compared to flanking, or arrows, etc. but it had to have happened enough that there were some ideas of how to do it properly...right?
I answered a very similar question a while ago in this thread, but the basic point is that movies and games lie to you about what premodern massed hand-to-hand combat was like. This summary:
asking a bunch of folks to run full-speed at a wall of sentient knives.
...is a decent account of what Rome: Total War suggests these battles were like, but that doesn't mean much for real history. If these battles really took the form of masses of men running headlong into each other, the ensuing chaos and carnage would indeed lead to massive casualties - which is precisely why scholars take it for granted that battles did not go like that. We have enough data on casualty rates in hoplite battles to know that they cannot have involved an opening stage of mass suicide.
Most likely, even if armies of spearmen would run to cover the last distance between them, they would slow down at the last moment in order to engage each other at spear range. Human beings don't willingly impale themselves and are far more likely to stop in their tracks and back away from the threat. Modern tests show that this is perfectly possible without causing collisions in massed formations; there is no meaningful intertia in a group of sentient individuals (unless they panic). Sources occasionally speak of spears breaking in the charge (especially with cavalry), but there is no evidence that men would bodily crash into each other. In battles where hoplites or legionaries encountered Hellenistic pikemen, they would not keep charging and run themselves onto the pikes like armoured shish kebab, but come to a halt and start a retreat until they were either driven off or disrupted the pike formation enough in its advance to be able to pounce on the gaps in the line.
You can read more in the posts linked above about how troops were motivated to do these things (since even this kind of tentative fighting was deadly and terrifying and regularly caused one side to flee before a single blow was struck). It was important to put the best and bravest in front, because otherwise the rest would not follow; but these front-rank positions were prestigious and coveted and clearly did not amount to a death sentence even if the battle was lost.