http://en.wikipedia.org/wiki/Blockade#Governing_laws
Soyez particulièrement attentif à la dernière phrase, Pegasus :
Governing laws
Whether or not a blockade was seen as lawful depended on the national
laws of the nations whose trade was influenced by the blockade. The Brazilian
blockade of Río de la Plata in 1826, for instance, was
considered lawful according to British law, but unlawful according to
French and American law. The latter two countries announced they would
actively defend their ships against Brazilian blockaders, while Britain
was forced to steer for a peaceful solution between Brazil and Argentina.[5]
Blockades were first defined in international law at the Congress of Paris in 1856. One of the
agreed rules was that a blockade had to be effective in order to be
lawful. This banned so-called « paper » blockades — blockades that were
declared to the blockaded nation, but were not actively enforced,
allowing the blockading party to seize the cargo of neutral states
trading with blockaded harbors.[6]
At the Declaration of London
in 1909 another attempt was made to further protect the rights of
neutral traders.[7]
The treaty was only ratified by a few nations, preventing any
application of the agreements. Parts of it were, however, applied during
blockades in World War I.
Since 1945, the UN Security Council
determines the legal status of blockades and by article 42 of the
UN Charter, the Council can also
apply blockades.[8]