It is on account of the practical importance of blockade for the interests of neutrals that the matter is more conveniently treated with neutrality than with war. And it must be noted that blockade as a means of warfare must not be confounded with so-called pacific blockade, which is a means of compulsive settlement of State differences.
Apart from the stipulation of the Declaration of Paris that a blockade to be binding must be effective, no conventional rules concerning blockade were in existence until the Declaration of London, nor was the practice of the States governed by common rules covering all the points concerned. But articles 1-21 of the Declaration of London now offer a code of the law of blockade and will, should this Declaration be ratified, in time produce a common practice of all maritime States.
Blockade, Strategic and Commercial.
§ 369. A blockade is termed strategic if it forms part of other military operations directed against the coast which is blockaded, or if it be declared in order to cut off supply to enemy forces on shore. In contradistinction to blockade strategic, one speaks of a commercial blockade, when a blockade is declared simply in order to cut off the coast from intercourse with the outside world, although no military operations take place on shore. That blockades commercial are, according to the present rules of International Law, as legitimate as blockades strategic, is not generally denied. But several writers[735] maintain that blockades purely commercial ought to be abolished as not in accordance with the guaranteed freedom of neutral commerce during war.
[735] See Hall, § 233.
Blockade to be Universal.
§ 370. A blockade is really in being when vessels of all nations are interdicted and prevented from ingress or egress. Blockade as a means of warfare is admissible only in the form of a universal blockade, that is—as article 5 of the Declaration of London stipulates—it "must be applied impartially to the vessels of all nations." If the blockading belligerent were to allow the ingress or egress of vessels of one nation, no blockade would exist.[736]
[736] The Rolla (1807), 6 C. Rob. 364; the Franciska (1855), Spinks, 287. See also below, § [382].
On the other hand, provided a blockade is universal, a special licence of ingress or egress may be given to a special vessel and for a particular purpose,[737] and men-of-war of all neutral nations may be allowed to pass to and fro unhindered.[738] Thus, when during the American Civil War the Federal Government blockaded the coast of the Confederate States, neutral men-of-war were not prevented from ingress and egress. But it must be specially observed that a belligerent has a right to prevent neutral men-of-war from passing through the line of blockade, and it is entirely within his discretion whether or not he will admit or exclude them; nor is he compelled to admit them all, even though he has admitted one or more of them.
[737] This exception to the general rule is not mentioned by the Declaration of London, but I have no doubt that the International Prize Court would recognise it.