Following the Russian rejection of a Japanese plan to divide Manchuria and Korea into spheres of influence, Japan launches a surprise naval attack against Port Arthur, a Russian naval base in China. The Russian fleet was decimated. During the subsequent Russo-Japanese War, 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 Japanese naval forces under Admiral Heihachiro Togo; 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 Togo near the Tsushima Islands. These three major defeats convinced Russia that further resistance against Japan's imperial designs for East Asia was hopeless, and in August 1905 U.S. President Theodore Roosevelt mediated a peace treaty at Portsmouth, New Hampshire. (He was later awarded the Nobel Peace Prize for this achievement.) Japan emerged from the conflict as the first modern non-Western world power and set its sights on greater imperial expansion. However, for Russia, its military's disastrous performance in the war was one of the immediate causes of the Russian Revolution of 1905.