But their order of importance is quite different when each is cooperating with the other two. Destruction being a more effective factor than mobility, the complete absence of cavalry would prove to be less debilitating to an army than the complete absence of artillery.
An army consisting only of infantry and artillery would, to be sure, find itself at a disadvantage when faced with one composed of all three arms. But if it were to make up for the missing cavalry by a proportionately stronger infantry force, a change in its tactical dispositions would enable it to manage fairly well. Outposts would pose some difficulties: there could be no brisk pursuit of a defeated enemy; and its own retreat would cause greater hardships and exertions. But such difficulties alone would hardly be sufficient to drive it off the field. If, on the other hand, such an army were faced with one composed only of infantry and cavalry it would stand up very well indeed. It is, in turn, almost inconceivable that the latter type could hold out at all against an army composed of all three arms.
It is understood that these reflections on the importance of each arm of the services are derived from the whole mass of military data, where one instance is analogous to another. It cannot be our intention to apply the facts we have discovered to every single phase of any given engagement. A battalion retreating or doing outpost duty would probably prefer some cavalry to a few guns. A body of cavalry and horse artillery with the task of pursuing a retreating enemy or cutting off his escape will find infantry completely useless, and so forth.
Let us recapitulate the results of these reflections:
1. Infantry is the most independent of the arms.
2. Artillery has no independence.
3. When one or more arms are combined, infantry is the most important of them.
4. Cavalry is the most easily dispensable arm.
5. A combination of all three confers the greatest strength.