In that respect, it is an act of war, and any attempt to exercise it against a power strong enough to resist would be a commencement of hostilities and at once bring into play the rights and duties affecting neutrals.
It is usual to refer to the intervention of France, Britain and Russia in Turkish affairs in 1827 as the first occasion on which the coercive value of pacific blockades was put to the test.
In 1850, Britain blockaded the ports of Greece in order to compel the Hellenic government to give satisfaction in the Don Pacifico case.
Some ill-defined measures of blockade followed such as that of 1860, when Victor Emmanuel, the king of Sardinia, joined the revolutionary government of Naples in blockading ports in Sicily, then held by the king of Naples, without any rupture of pacific relations between the two governments; that of 1862 in which Britain blockaded the port of Rio de Janeiro to exact redress for pillage of a British vessel by the local population and at the same time declaring that it continued to be on friendly terms with the emperor of Brazil; and that in 1880, when a demonstration was made before the port of Ulcinj by a fleet of British, German, French, Austrian, Russian and Italian men-of-war, to compel the Ottoman government to carry out the treaty conceding the town to Montenegro, and it was announced that if the town was not given up, by the Ottoman forces it would be blockaded.
The British government held that in those circumstances, France was waging war and not entitled to combine the rights of peace and warfare for her own benefit.
Since then, pacific blockades have been only exercised by the great powers as a joint measure in their common interest, which has also been that of peace; and in this respect the term is taking a new signification in accordance with the ordinary sense of the word 'pacific'.
In 1886, Greece was blockaded by Britain, Austria, Germany, Italy and Russia to prevent it from engaging in war with the Ottoman Empire and thus forcing the powers to define their attitude towards it.
That blockade was not pacific but was war with all its consequences for belligerents and neutrals (see Foreign Office notice in London Gazette of December 20, 1902).