Initially allied with Great Britain through the Anglo-Japanese Alliance, Japan viewed the outbreak of World War I as an opportunity to assert its dominance in the region. On August 23, 1914, the Japanese government formally declared war on Germany after the latter failed to accept its ultimatum to withdraw from German-occupied territories in China, specifically in Shandong Province. Japan aimed to occupy the German-held Port Arthur, vital for naval power and operations in the region, as well as other territories, including islands in the Pacific. Japan's military quickly mobilized, and by the end of the year, they had seized the German concession at Tsingtao and other important positions in the region.

The declaration of war was part of Japan's broader strategy to strengthen its presence in Asia while European nations were distracted. The decision also marked a shift in Japan's global role, as it transitioned from a nation primarily focused on domestic issues and regional conflicts to one actively participating in international military affairs. The military campaign was characterized by effective Japanese naval operations, which secured supremacy over German naval assets in the Pacific. In addition, Japan’s involvement in the war significantly enhanced its diplomatic standing with the Western powers.

Post-war, Japan was included in the negotiations at the Treaty of Versailles, leading to several territorial gains. Nonetheless, Japan's growing militarism and the aims for further expansion would lay the foundations for conflict in Asia during the interwar years and eventually lead to its central role in World War II. Japan's participation in World War I is often regarded as pivotal in its emergence as a major power on the global stage and set the course for its future military endeavors.