Russo-Japanese War, 1904-5, imperialistic conflict that grew out of the rival designs of Russia and Japan on Manchuria and Korea. Russian failure to withdraw from Manchuria and Russian penetration into N Korea were countered by Japanese attempts to negotiate a division of the area into spheres of influence. The Russian government, however, was inflexible, and it was willing to risk an armed conflict in the belief that Japan was bound to be defeated and that a Russian victory would head off the growing threat of internal revolution in Russia. Japan broke off negotiations and severed (Feb. 6, 1904) diplomatic relations with Russia. Two days later, without a declaration of war, Japan attacked Port Arthur and bottled up the Russian fleet. A series of quick Japanese victories, which astounded the world, culminated in the fall of Port Arthur (Jan., 1905), the victory of troops under General Oyama at Shenyang (Feb.-Mar., 1905), and the destruction of the Russian fleet under Rozhdestvenski at Tsushima by Admiral Togo's fleet (May, 1905). Through the mediation of U.S. President Theodore Roosevelt, peace was made in September at Portsmouth, N.H. (see Portsmouth, Treaty of). The disastrous outcome of the war for Russia was one of the immediate causes of the Russian Revolution of 1905. Japan gained the position of a world power, becoming the first non-European and non-American imperialist modern state.

(1904–05) Conflict between Russia and Japan over territorial expansion in East Asia. After Russia leased the strategically important Port Arthur (now Lüshun, China) and expanded into Manchuria (northeastern China), it faced the increasing power of Japan. When Russia reneged on its agreement with Japan to withdraw troops from Manchuria, the Japanese fleet attacked the Russia naval squadron at Port Arthur and began a siege of the city in February 1904. Japanese land forces cut the Russian army off from coming to aid Port Arthur and pushed it back to Mukden (now Shenyang). The reinforced Russian army took the offensive in October, but poor military leadership blunted its effectiveness. After the long Japanese siege of Port Arthur, in January 1905 the corrupt Russian commander surrendered the garrison without consulting his officers, despite adequate stores and ammunition for its continued defense. Heavy fighting around Mukden ended in March 1905 with the withdrawal of Russian troops under Aleksey Kuropatkin. The decisive naval Battle of Tsushima gave the Japanese the upper hand and brought Russia to the peace table. With the signing of the Treaty of Portsmouth, Russia abandoned its expansionist policy in eastern Asia and Japan gained effective control of Korea and much of Manchuria.

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