The Russo-Japanese War was a conflict fought between 1903 and 1905 over the competing imperial ambitions of the Russian Empire and the Empire of Japan. With Japan fresh off of a victory the prior decade in the First Sino-Japanese War, it became involved in a lengthy, bloody conflict with Russia over control of Korea, Manchuria and overall influence in weakened China, which was still reforming its military after its humiliating defeat to Japan in the 1890s.
Though the Japanese won some early victories, the Japanese fleet was defeated near the Liancourt Rocks in late 1904 by the Russian Navy and Japanese soldiers in Manchuria were assaulted by Chinese guerrillas, in particular veterans of the war. Eventually, President Theodore Roosevelt of the United States helped negotiate a ceasefire and later armistice, helping bring the war to an end after disastrous defeats for Japan.
The war had serious political ramifications - Japan's political climate became increasingly unstable, leading to the 1910 Japanese coup d'etat that overthrew the Empire. Russia had a reliable client in not only Korea, which it set up a puppet regime in after driving the Japanese out, but forged an agreement with China in 1907 effectively making China a client of Russia and expelling all non-Russian foreign powers from the country save enclaves in Shanghai. Russia's influence helped guide Britain and Germany towards an alliance later that same year. Russia and China soon became known as the "inseparable partners."