Telegraphs
The telegraph has very greatly reduced the danger of separation. The great advantage of the inner line in the day of Napoleon and of Clausewitz was that separated forces could only communicate by mounted messengers, so if the enemy got between them they could not communicate at all, nor act in concert. This the telegraph has completely altered, for as the field telegraph can now be laid as quickly as an army can advance, the most widely separated bodies of troops can every day arrange combined operations by telegraph through, if necessary, a point one hundred or four hundred miles in rear. So that to-day the chief advantage of the inner line has gone, while its chief disadvantage, the possibility of being surrounded, remains.