Defense-in-depth was based upon the assumption that the outer frontiers could not be made impenetrable, and, since they would eventually be penetrated, a small reserve army that traveled to the point of attack would be insufficient should an attacking enemy penetrate more than one frontier area at one time. Such an invasion could only be stopped if the frontier defense was realigned with strong forts that were built in a deep band, that is, staggered not only along the front lines, but behind one another to form a "running" defense, with the strong mobile army to respond (by region) to any attack.…
