What happened
The war opened in February 1904 with Japan’s surprise attack on the Russian fleet at Port Arthur. The siege of Port Arthur, which fell in January 1905, cost enormous casualties, and Mukden (February–March 1905) was the largest land battle fought to that date. At Tsushima on May 27–28, 1905, Admiral Togo’s fleet annihilated Russia’s Baltic squadron.
The Treaty of Portsmouth (September 5, 1905), mediated by US President Theodore Roosevelt, gave Japan the Liaodong leasehold, the South Manchuria Railway, southern Sakhalin, and a free hand in Korea — but no indemnity, sparking the Hibiya riots in Tokyo.
Background
Japan and Russia had been contending for dominance in Manchuria and Korea.
Consequences
It was the first modern defeat of a European great power by an Asian state, noted by colonized peoples worldwide. It was also a decisive step in Japan’s continental expansion; Japan annexed Korea in 1910.