DEPTH, in military language, the space over which a body of troops is distributed from front to rear. In a tactical sense it is used to express the idea of, and need for, so distributing a force or unit as to have adequate reserves behind the fighting line to exploit initial success or to meet an emergency. In the World War the tendency was for the distribution of a force or unit to attain an ever-increasing depth in proportion to, and with a proportion ate reduction of, its frontage.