The First Sino-Japanese War (1894–95) has been perceived as a sign of a new East Asian power order, but the legitimacy of the war has yet to be clarified. The Japanese foreign minister Mutsu’s Kenkenroku shows that the reasons claimed by Japan were only pretexts for its ambition to put Korea under its control. The 1885 Convention of Tianjin, which was used to justify the Japanese behaviour, needs to be reinterpreted. The Chinese reaction can be understood by exploration into Confucianism, which opposed wars between equal peers. Meanwhile, the Western powers which invented and developed international law were self-interested and did little to prevent the war. The incident shows that international law, empowered by the strong states, failed to maintain peace efficiently in the late nineteenth century.