World War I

In 1914 Japan joined Great Britain and the other Allies in the war against Germany, primarily because of the Anglo-Japanese alliance of 1902. Japanese troops occupied German colonies on the Kiaochow Peninsula of China.

After the Russian Revolution of 1917, Japan joined an Allied expedition to Siberia against the Communists. The campaign soon developed into a stalemate.

The Treaty of Versailles gave Japan the Kiaochow Peninsula and the former German islands in the Pacific, excepting German Samoa. At the Washington Naval Conference in 1921, Japan agreed to return the Kiaochow Peninsula to the Chinese, to withdraw from Russian Siberia, and to reduce its naval armament. In 1926, Hirohito succeeded to the throne.

Japan participated in the Three Power Conference at Geneva in 1927 and was a party to the London Naval Treaty of 1930. It appeared that the Japanese had given up expansion on the Asian mainland. However, a military group had been gaining power in the Japanese government.