1) Japan expanded into Korea and Manchuria in the early 1900s seeking new markets and resources as well as to demonstrate its strength to Western powers.
2) Japan allied with Britain in 1902 due to shared interests in countering Russian expansion, and defeated Russia in the 1904-1905 Russo-Japanese War, gaining control of Korea and influence in Manchuria.
3) Rapid modernization and industrialization created social and economic problems for Japan, and its military grew more influential in politics seeking to establish Japanese dominance in Asia.