During the Russo-Japanese War, Port Arthur, the Russian naval base in China, falls to Japanese naval forces under Admiral Heihachiro Togo. It was the first in a series of defeats that by June turned the tide of the imperial conflict irrevocably against Russia.
In February 1904, following a Russian rejection of a Japanese plan to divide Manchuria and Korea into spheres of influence, Japan launched a surprise naval attack on Port Arthur, decimating the Russian fleet. In the subsequent fighting, Japan won a series of decisive victories over the Russians, who underestimated the military potential of its non-Western opponent.
In January 1905, the strategic naval base of Port Arthur fell to the Japanese; in March, Russian troops were defeated at Shenyang, China, by Japanese Field Marshal Iwao Oyama; and in May, the Russian Baltic fleet under Admiral Zinovi Rozhdestvenski was destroyed by Admiral Togo's fleet near the Tsushima Islands. These three crucial defeats convinced Russia that further resistance against Japan's imperial designs on East Asia was hopeless, and in August 1905 President Theodore Roosevelt mediated a peace treaty at Portsmouth, New Hampshire.
Japan emerged from the conflict as the first modern non-Western world power and set its sights on greater imperial expansion. For Russia, however, the disastrous performance in the war was one of the immediate causes of the Russian Revolution of 1905.