5. Lastly, on a retreat, as rearguard, to be used in defending the principal natural obstacles of ground. In this respect also the centre is exceedingly important. At first sight it certainly appears as if such a rearguard would be constantly in danger of having its flanks turned. But we must remember that, even if the enemy succeeds in overlapping the flanks to some extent, he has still to march the whole way from there to the centre before he can seriously threaten the central mass, which gives time to the rearguard of the centre to prolong its resistance, and remain in rear somewhat longer. On the other hand, the situation becomes at once critical if the centre falls back quicker than the wings; there is immediately an appearance as if the line had been broken through, and even the very idea or appearance of that is to be dreaded. At no time is there a greater necessity for concentration and holding together, and at no time is this more sensibly felt by every one than on a retreat. The intention always is, that the wings in case of extremity should close upon the centre; and if, on account of subsistence and roads, the retreat has to be made on a considerable width (of country), still the movement generally ends by a concentration on the centre. If we add to these considerations also this one, that the enemy usually advances with his principal force in the centre and with the greatest energy against the centre, we must perceive that the rear guard of the centre is of special importance. Accordingly, therefore, a special corps should always be thrown forward as an advanced guard in every case where one of the above relations occurs. These relations almost fall to the ground if the centre is not stronger than the wings, as, for example, Macdonald when he advanced against Blücher, in Silesia, in 1813, and the latter, when he made his movement towards the Elbe. Both of them had three corps, which usually moved in three columns by different roads, the heads of the columns in line. On this account no mention is made of their having had advanced guards. But this disposition in three columns of equal strength is one which is by no means to be recommended, partly on that account, and also because the division of a whole army into three parts makes it very unmanageable, as stated in the fifth chapter of the third book.