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Manual of the Laws of Naval War. Oxford, 9 August 1913.
Section V : On the rights and duties of the belligerent with regard to individuals - Art. 56.
Art. 56. Public or private vessels. When an enemy ship, public or private, is seized by a belligerent, such of its crew as are nationals of a neutral State, are not made prisoners of war. The same rule applies in the case of the captain and officers likewise nationals of a neutral State, if they promise in writing not to take, during hostilities, any service connected with the operations of the war. The captain, officers and members of the crew, when nationals of the enemy State, are not made prisoners of war, on condition that they make a formal promise in writing not to undertake, while hostilities last, any service connected with the operations of the war.