Exceptions to this as to all strategical rules may be conceived. Conditions might exist in which, if the enemy's battle-fleet accompanied his transports, it would be worth our while, for ulterior objects of our own, to risk the escape of the transports in order to seize the opportunity of destroying the fleet. But even in such a case the distinction would be little more than academical; for our best chance of securing a decisive tactical advantage against the enemy's fleet would usually be to compel it to conform to our movements by threatening an attack on the transports. It is well known that it is in the embarrassment arising from the presence of transports that lies the special weakness of a fleet in charge of them. There is, however, one condition which radically differentiates comparatively small expeditions from great invasions and that is the power of evasion. Our experience has proved beyond dispute that the navy alone cannot guarantee defence against such expeditions. It cannot be sure of preventing their sailing or of attacking them in transit, and this is especially the case where an open sea gives them a free choice of route, as in the case of the French expeditions against Ireland. It is for this reason that, although an adequate navy has always proved sufficient to prevent an invasion, for defence against expeditions it must be supplemented by a home army. To perfect our defence, or, in other words, our power of attack, such an army must be adequate to ensure that all expeditions small enough to evade the fleet shall do no effective harm when they land. If in numbers, training, organisation, and distribution it is adequate for this purpose, an enemy cannot hope to affect the issue of the war except by raising his expeditions to invasion strength, and so finding himself involved in a problem that no one has ever yet solved for an uncommanded sea.