No rule of international law existed before the adoption of the present Convention preventing belligerent States from confiscating enemy merchantmen in their harbour or on the high seas at the outbreak of a war. From the beginning of the Crimean War, in 1854, it became the usage followed by some countries that enemy vessels in the harbours of a belligerent should be granted a reasonable time to depart unmolested and that such vessels, if met at sea, should be permitted to continue their voyage. It was, however, not before the Second Hague Peace Conference that binding rules to this effect were adopted.
D.Schindler and J.Toman, The Laws of Armed Conflicts, Martinus Nijhoff Publisher, 1988, pp.791-794.