The effect of an army is both material and moral, Colonel Ardant Du Picq says:
The material effect of an organization is in its power to destroy, the moral effect in the fear that it inspires.
In battle, two moral forces, even more than two material forces, are in conflict. The stronger conquers. The victor has often lost by fire more than the vanquished. Moral effect does not come entirely from destructive power, real and effective as it may be. It comes, above all, from its presumed, threatening power, present in the form of reserves threatening to renew the battle, of troops that appear on the flank, even of a determined frontal attack.
Material effect is greater as instruments are better (weapons, mounts, etc.), as the men know better how to use them, and as the men are more numerous and stronger, so that in case of success they can carry on longer.
With equal or even inferior power of destruction he will win who has the resolution to advance, who by his formations and maneuvers can continually threaten his adversary with a new phase of material action, who, in a word has the moral ascendancy. Moral effect inspires fear. Fear must be changed to terror in order to vanquish.
When confidence is placed in superiority of material means, valuable as they are against an enemy at a distance, it may be betrayed by the actions of the enemy. If he closes with you in spite of your superiority in means of destruction, the morale of the enemy mounts with the loss of your confidence. His morale dominates yours. You flee. Entrenched troops give way in this manner.