Japan entered World War I as a member of the Allies on 23 August 1914, seizing the opportunity of Imperial Germany's distraction with the European War to expand its sphere of influence in China and the Pacific. There was minimal fighting. Japan already had a military alliance with Britain, but that did not obligate it to enter the war. It joined the Allies in order to make territorial gains. It acquired Germany's scattered small holdings in the Pacific and on the coast of China.

Historians writing about the origins of World War I have differed over the relative emphasis they place upon the factors involved. Changes in historical arguments over time are in part related to the delayed availability of classified historical archives. The deepest distinction among historians remains between those who focus on the actions of Germany and Austria-Hungary as key and those who focus on a wider group of actors. Meanwhile some historians, such as Fritz Fischer, maintain that Germany deliberately sought war while others do not. The main distinction among the latter is between those who believe that a war between the "Great Powers" was ultimately unplanned but still caused principally by Germany and Austria-Hungary taking risks, and those who believe that either all or some of the other powers, namely Russia, France, Serbia and Great Britain, played a more significant role in risking war than had been traditionally suggested.