This demands that victory be won in the shortest possible time, since the cost of war is nearly proportional to the duration of the conflict. Moreover, in the prosecution of the conflict itself, victory comes to the combatant who can most rapidly mass his resources of men and materials at the critical points. It is for these reasons that time is so important an element in all military operations, including construction.

Waste not Justifiable

The achievement of our purpose in war calls for a rapid expenditure of both life and material. It is the duty of the Commander, however, to expend in the most economical manner possible the resources which the nation by painful sacrifice has placed at his disposal; and the same obligation rests upon all his subordinates. But great expenditures to accomplish great results are not wasteful.

The engineer must thoroughly disabuse his mind of any belief that military necessity ever calls for or justifies waste of the nation's resources; but he must not allow a penny-wise inclination toward economy of material or money to cause him to overlook the greater necessity for economy of time. Material should be freely expended to save time, but it should never be wasted. However great the resources of the nation, there is always a dearth of construction material in war, due not alone to lack of material but also to lack of transportation facilities for delivering it at the places where it is required. A reckless use of material in one locality may mean the failure of important operations in some other place where the material thus wasted is needed. A small saving of material must not be made at the expense of a great waste of time; but the soldier who deliber- ately or carelessly wastes any useful material is guilty of highly unpatriotic, not to say criminal, conduct.

Classes of Military Bridges

There are two general classes of military bridges;

(a) Those constructed in rear of the battle lines, in the zone of communications, not in the immediate presence of the enemy.

(b) Those constructed at the front, within the combat zone or the area subject to hostile fire and raids.

Structures of the first class are erected for purposes and under conditions approximating those of civil construction in time of peace. The need for such bridges is determined mainly from strategical considerations, and they seldom have any intimate relation to the tactical operations of the combatant forces. As compared to bridges within the zone of tactical activities, they are usually needed for a relatively long period—from several months to the duration of the war. They are required to carry heavy loads. They may be constructed by non-combatant troops, by hired civilian labor, or even by contract. Construction plant, similar to that employed in civil works,  will  be  utilized  whenever  it  is  available. With  the  exception