In a repetition of a pattern of external forces dictating the pace of internal change, the First World War, a many-sided opportunity rather than a threat, enlarged Japan's external involvement to a scale which could not have been anticipated before 1914, and ultimately, through the complications that followed, led disastrously into what the Japanese were to call the Great Pacific War (1941–5). Once the 1914–18 war began, Britain wished for Japanese support, though hoping to limit Japan's role to action at sea against Germany rather than territorial acquisition. However, Japan went on to occupy the German possessions in Shantung and the German islands in the north Pacific. That far, Japanese involvement, if more than Britain relished, was at least in line with the conventional action of a great power.

While occupation of Shantung was defensible on the principle of realpolitik, the Twenty-One demands made by Japan in 1915 on the Chinese government, whose terms, if conceded, would have involved a measure of political control of China, were a different matter. The demands failed to recognise Chinese nationalism, which, already growing, had become a new force, and represented a potential derailment of Japanese foreign policy. They were denounced by the senior genrō, Yamagata, and the objectionable political demands were dropped from the programme (reduced to fifteen demands). On the narrowest criterion, that is, ignoring Chinese rights altogether (the western standard since 1840), the Twenty-One demands were incoherent, at one and the same time an implicit challenge to rival imperial powers and an impulsive formulation of policy by faction rather than by deliberation.