Now, as it happens, there has been a certain amount of interest in this subject in military circles recently and I've read some of the literature on this subject:
The Stress of Battle
The Human Face of War
Brains and Bullets
Men Against Fire
Plus some stuff in military journals and it appears that there are certain factors which can be identified and their importance given a very provisional numerical rating. So I have done a very imperfect summary of the factors identified as making the difference:
Some Notes on Morale, or Why Soldiers Don’t Fight
“It is a fact that scattered through the Army there is a great deal of experience and knowledge of the problems of morale. But it has never been collected or systematised. Everybody’s opinion depends on his own experience and observation, which may or may not be representative.
The result is that any discussion of morale sharply divides into two stages:
1. The stage of woolly abstractions in which people talk solemnly of ‘leadership’ and ‘discipline’ or ‘group spirit’ without ever defining the meaning of these phrases in practice;
2. The all-too-concrete stage, in which the whole subject suddenly degenerates into discussions about supplies of beer.” (Brigadier Nigel Balchin).
What Makes Soldiers Not Fight:
1. Being Surprised. Surprise seems to be the single greatest combat multiplier available to a commander. Poor troops who are surprised are almost inevitably defeated, but all troops are likely to be. Panic and flight are the most likely outcomes. Obviously surprise wears off quite quickly. Within limits, troops who are surprised can be defeated by far inferior forces (numerically).
2. Being Shocked… Troops in a state of shock are likely to fight very poorly, if it all, and may put up literally no resistance. This could be achieved by bombardment, aerial attack, constant attack or defeat(ism). Units and individuals do recover from shock, sometimes in a relatively short period, depending on the nature of the shock. Longer-term effects seem to be a feature only of C20+ warfare (there was nowhere near enough fire to achieve these effects before that).
3. …And Being Suppressed. On a tactical level, being suppressed seems to be a form of the same thing; but at the most basic level i.e. troops recover from suppression very quickly when the suppressive fire stops. There is an echo of this in the way that the spirits of allied soldiers at Waterloo improved when they were about to be attacked by cavalry, as the artillery fire was stopped. Also consider the crews of Tiger tanks who abandoned their tanks after coming under artillery bombardment or Typhoon attack.
4. Lack of Supervision and Leadership/Compulsion. Soldiers who are not supervised are not likely to fight well in difficult situations, being reluctant to close with the enemy or fight as hard for ground. This is not a matter of courage per se but more a matter of isolation affecting judgement of risk and reward. Conversely, soldiers who are supervised by higher ranks are much more likely to perform well, even to maximal standards. This is the basic reason why anti-tank guns generally outperform tanks in combat: a tank is a lonely, isolated place and a tank commander is a lonely and isolated individual. The Napoleonic emphasis on the courage of officers and their leading by personal example clearly taps into the importance of this. Discipline is another element of this – leaders need their troops to do what they tell them to do and units without that discipline are likely to come apart. Troops can sometimes do incredible things simply because the boss is at their shoulder, watching...
5. Cohesion. Sometimes it is cohesion, sometimes the lack of it. In large bodies of men operating together, cohesion, along with leadership/supervision, keeps soldiers in the ranks and obeying orders. This is why commanders were reluctant to give up close-order formations. Armies/unit types which do not achieve this cohesion have trouble getting close to the enemy and will prefer to skirmish. Melee combat between skirmishers is likely to be a desultory affair, with a couple of extraordinarily motivated individuals doing all the close-fighting, perhaps dragging a few people with them – so-called ‘Heroic Fighting’, but most of the soldiers will be fighting just outside effective range (or hanging back even further). In close order units however, when some men break, the unit’s cohesion will work against it and the whole unit is likely to rout, as soldiers under no immediate threat copy the behaviour of those in the unit who are.
6. Weapon/Equipment Factors. Troops perceiving themselves to have better weapons than the enemy and their fellows will fight harder. Troops in the reverse situation will fight less hard. This is particularly so when troops do not feel they have adequate weapons to respond (so troops without specific anti-tank or anti-aircraft weapons for example – the so-called tank panic and dive-bomber terror, etc). In general, this is why troops fear indirect fire and mines/booby-traps more than anything else.
7. Confusion. Troops confronted with a variety of different threats can have difficulty in doing anything at all as they or their commanders’ brains are overloaded with threats so they can’t think straight.
8. Being outflanked. Being outflanked, even without being surprised or shocked or confused (although all of these things are likely), is more likely to force an enemy to run away/surrender, partly for physical reasons (can’t bring weapons and immediate reserves to bear), but seems to have a psychological effect of its own too. Removing this threat – and the increase of supervision – is the chief value of the square.
9. Being given a credible alternative. If there is a way to escape imminent death/disaster, then troops will take it. Troops with no credible opportunity to run away or surrender are likely to fight harder.
10. Not being a hero. There are some people who naturally fight harder than others and some who will go quite a way to not fight at all. The majority will respond to leadership and be dependent upon it. Effective training and selection can reduce the size of the group reluctant to fight at all.
11. Aversion. Troops are simply less willing to engage in combat than supposed, whether from reluctance to hurt others or by abstaining from activity, hope to discourage enemy activity coming their way. This effect is quite large, particularly at close ranges or in hand-to-hand combat. On the other hand, well-trained elite troops not subjected to any particular effective fire from an enemy who they think will torture or kill them if they surrender are likely to have an aversion level close to nought. Supervision and cohesion probably help a lot in pre C20 battles, in the stages where commanders retain control and the visibility is reasonable.
12. Struggling to Close with the Enemy. There appears to be a range, just outside the effective range of enemy action/fire that people need a special motivational push to get beyond or to stay fighting at (if they are defending). Troops that get through this are more likely to go all the way (to close), or to hold. It appears to be the mechanism that prevented infantry units clashing in the horse-and-musket era.
13. Fussing. Generally a thing in modern combat – prioritizing looking after equipment/weapons etc. rather than fighting directly. A kind of displacement activity, but the muskets collected unfired with multiple charges inside them point to something similar. May be seen in commanders at all levels in all periods, failing to focus on the fighting and allowing themselves to be distracted, a relative of confusion.
14. Advertising. Troops can be scared into running off or surrendering if it is made clear to them that something very nasty is coming their way, but they have an easy way to avoid it – either an easy withdrawal route or someone kind to surrender too. Done at key moments then this is almost irresistible.
Obviously there is a lot more to all of this - in particular points 2 and 3 - but the numbers behind it do seem to stack up.