Sydney incident (シドニー号事件)
This incident occurred between Japan and France during The Sino-Japanese War
This incident ultimately set one example in the application of international law in time of war, as a passenger from a neutral country on a neutral ship was arrested as a "forbidden" person in wartime.
In October 1894, during The Sino-Japanese War, two American passengers called Wild and Cameron, along with Chinese legation staff of Washington, arrived at Yokohama on a vessel, Gehrig.
Because Japanese officials found out that these two were producers of underwater mines and had an intention of attacking Japanese Navy by their contract with Chinese government, they conducted a search on the vessel, only to find out that the two were already aboard a French vessel, Sydney, bounded for Shanghai via Kobe.
Japanese Navy again investigated the Sydney at Kobe and discovered some material evidences; two American were thus arrested and the fleet was released.
Although captain of fleet as well as the French consulate proclaimed the protest for this exceptional action, and there were many arguments amont many international law experts, this incident pioneered an example of punishing not only those who are obviously on military duty of hostile country, but also those who merely intend to be involved in.