It is, accordingly, a fundamental economic principle of military construction that bare necessities alone should be provided for. An engineer who wastes valuable time and material constructing a bridge wide enough and strong enough to carry heavy motor trucks, when the need of the instant is a simple foot-bridge over which a body of infantry may pass at once to a critical point of the field, has manifestly failed to grasp this fundamental economic principle.

Safety and Permanence

The civil engineer "builds for posterity." Whatever the type of bridge adopted, it is usually constructed in as enduring a fashion as the funds available and the material employed will permit. The useful life of the bridge is ordinarily a measure of the skill of its builder.

The military engineer builds to meet the exigency of the moment. He is lacking in skill if he expends time in order that his bridge may endure unduly long beyond the period it is needed, which in no case exceeds the duration of the war, and is often limited to that of a single action. The thoroughness of his work should be sufficient unto the immediate needs—and no more than sufficient. Nicety, finish, refinement, and permanency, for their own sake, are to be avoided. The military bridge should have no beauty except that which is inherent in utilitarianism.

In civil construction great weight is properly given to the factor of safety and to the durability or permanence of the structure. In military procedure these considerations have far less weight. The factor of safety need seldom be as great, although this will depend to some extent on the situation. Where a military force is entirely dependent on a single line of supply, the bridges on this line should have a factor of safety as great or nearly as great, as would be employed in civil practice. But ordinarily, in the combat zone at least, the saving in time and material which results from using a factor of safety of 2 instead of 4 or 5 will more than compensate the risk involved in the possible collapse of structures. The risks attendant upon military operations are so numerous and so great that the slight additional risk of a small factor of safety is of minor significance.

Permanence is of negligible importance in military construction, inasmuch as the structures will never be required beyond the duration of the war, and frequently for much shorter periods. It is generally good economy first to meet the exigency of the moment, and later to repair, strengthen, or even replace the structure, should this be necessary by reason of continued need. The time for which any military structure will be required is usually short, and always uncertain, so that it is not good economy to look too far into the future.

Warfare is an economic art, no less than any of the pursuits of peace. In war we have a mission to perform, which is the achievement of victory; and this mission should be accomplished with the least possible expenditure of blood and treasure — in other words, in the most economical fashion.