Railways
"Railways have, above all, completely altered the term 'base,'" remarks V. Caemmerer. "Railways carry in a few days men, horses, vehicles, and materials of all kinds from the remotest district to any desired point of our country, and nobody would any longer think of accumulating enormous supplies of all kinds at certain fortified points on his own frontier with the object of basing himself on those points. One does not base one's self any longer on a distinct district which is specially prepared for that object, but upon the whole country, which has become one single magazine, with separate store-rooms. So the term 'base' has now to be considered in this light."
It is only when operating in savage or semi-savage countries, where there are no railways, that the old idea of a base applies.
As we penetrate deeper and further from our own country into the enemy's, and as a small raiding party can demolish the railway line so as to stop all traffic for days or weeks, it becomes far more necessary than it ever was in Clausewitz's day to guard our communications. And armies become more and more sensitive to any attack upon their communications.
Also "such a line cannot easily be changed, and consequently those celebrated changes of the line of communication in an enemy's country which Napoleon himself, on some occasion, declared to be the ablest manœuvre in the art of war, could scarcely be carried out any more" (V. Caemmerer).
Also concentration by means of several railways demands a broad strategic front, which produces that separation of corps or armies which prepares the way for strategical envelopment, and so on.
General von der Goltz, in his "Conduct of War," says: "The more recent treatises on the conduct of war on a large scale are principally taken up with the mobilization and strategical concentration of armies, a department of strategy which only began to play an important part in modern times. It is the result of a dense net-work of railways in Western Europe which has rendered it possible to mass large bodies of troops in a surprisingly brief time. Each Power tries to outdo its neighbours in this respect, ... which gives an opportunity to the strategical specialist to show off his brilliant qualities.... Consequently it is now frequently assumed that the whole conduct of war is comprised in this one section of it." This over-estimate is of course an error, which, however, requires to be pointed out.