There is, moreover, one characteristic of naval warfare which has no exact counterpart in the conduct of military enterprises on land. This is the power which a naval belligerent has of withdrawing his sea-going force out of the reach of the sea-going force of the enemy by placing it in sheltered harbours too strongly fortified for the enemy to reduce by naval power alone. The only effective answer to this which the superior belligerent can make is, as has already been shown, to establish a blockade of the ports in question. This procedure is analogous to, but not identical with, the investment by military forces of a fortress in which an army has found shelter in the interior of the enemy's country. But the essential difference is that the land fortress can be completely invested so that no food or other supplies can reach it, whereas a sea fortress cannot, unless it is situated on a small island, be completely invested by naval force alone. In the one case, even if no assault is attempted, starvation must sooner or later bring about the surrender of the fortress together with any military force it contains, whereas in the other the blockaded port being, as a rule, in open communication with its own national territory, cannot be reduced by starvation. Moreover, for reasons already explained, a maritime fortress cannot nowadays be so closely blockaded as to prevent the exit of small craft almost at all times or even to prevent the exit of squadrons of battleships in circumstances favourable to the enterprise. Now the exit of small craft equipped for torpedo attack is a much more serious threat to the blockader than the exit of small craft, not so equipped, was in the old days of close blockade. In those days small craft could do no harm to ships of the line or even to frigates, whereas a torpedo craft is nowadays in certain circumstances the equal and more than the equal of a battleship. For these reasons the escape from a blockaded port of a squadron of battleships might easily be regarded by the blockading enemy as a less serious and even much more welcome incident of the campaign than the frequent issue of swarms of torpedo craft skilfully handled, daringly navigated, and sternly resolved to do or die in the attempt to reduce the battle superiority of the enemy.