Russo-Japanese War, armed conflict between Russia and Japan in 1904-5. The cause of the war was that Russian expansion in eastern Asia ran counter to Japanese plans for gaining a foothold on the Asian mainland. In 1898 Russia leased Port Arthur (now Lüshun) from China, with the intention of making it a great Asiatic port and the headquarters of Russian naval power in the Pacific. Russia had poured troops into Manchuria during the Boxer Uprising in 1900 but, faced with the Anglo-Japanese alliance of 1902, promised to leave Chinese territory. The promise was not kept, however, and in June 1903 Japan proposed an agreement with Russia recognizing Japan's interests in Korea and Russia's in Manchuria, and insuring the integrity of China and Korea. Russia refused. On the night of February 8, 1904, the Japanese navy launched a surprise attack on Port Arthur and then blockaded the damaged Russian fleet.

In March 1904, the Japanese First Army landed at Chemulp’o (now Inch’ǒn) and at Namp’o. It faced the Russians on the Yalu River by late April. The Russian commander in chief, General Aleksey Nikolayevich Kuropatkin, decided to hold a defensive position on the Liaodong Peninsula until enough reinforcements arrived to permit him to move to the offensive. His forces were spread out over a distance of 24 km (15 mi). The Japanese commander, General Tamemoto Tamesada Kuroki, built up a 4-1 troop advantage at the town of Uiju and forced a passage across the Yalu on May 1. Japanese losses were about 1100 out of 40,000 men. The Russians lost some 2300 out of 7000 troops.

During May the Japanese Second Army under General Yasukata Oku landed on the Liaodong Peninsula, severed communications between Port Arthur and the main Russian forces in Manchuria, captured Kinchow (now Jing Xian, or Ching Hsien), won the Battle of Nanshan (May 30), and besieged Port Arthur. The Japanese Third Army under General Maresuke Nogi landed on the eastern coast of Korea, and the Fourth Army under General Michitsura Nodzu landed at Gushan (Kushan) on the Manchurian coast. Kuropatkin was personally ordered by Nicholas II, emperor of Russia, to hold Port Arthur, and he moved his main forces to Liaoyang. After defeating a Russian force of 25,000 at Wafangdian (Wafangtien) on June 14, the Japanese began to converge on Liaoyang. Kuropatkin attacked Kuroki's forces in late July and then fell back to Liaoyang. Between August 25 and September 4, the Battle of Liaoyang was fought. Although the Japanese were outnumbered 130,000 to 180,000, they defeated the Russians, who retreated toward Shenyang (Mukden).

The Japanese campaign had been successful, but their reserves of trained men were rapidly being exhausted. On the other side, Kuropatkin's army was being reinforced by about 30,000 men a month via the Trans-Siberian Railroad, and the commander in chief now decided to attack. The first Russian offensive was a river assault launched on the Sha He (Sha Ho), lasting from October 5 to October 17, and a second attack at Sandepu on January 26-27, 1905. Both offensives broke down, however, and were indecisive.

Time favored Russia, and the Japanese, who had been besieging Port Arthur since May, once more tried to storm it. After a bloody assault lasting ten days, in which they lost 10,000 men, the Japanese captured the key position, called 203 Meter Hill, on December 5, 1904. The Russian general Anatoly Mikhailovich Stësel surrendered on January 2, 1905.

Now both Kuropatkin and the Japanese commander, Marshal Iwao Oyama , were able to build up their forces around Shenyang. Between February 19 and March 10 the Japanese took the offensive. Although their forces were outnumbered 270,000 to 330,000 and were inferior in artillery, the Japanese forced the Russians to surrender Shenyang and withdraw northward. The Russians lost some 90,000 men and the Japanese about 50,000. The battle practically ended hostilities on land, but Japan still faced the threat of Russian naval power.

With their Far East naval forces contained by the Japanese, the Russians decided to send out the Baltic Fleet. The fleet, made up of 45 ships, sailed October 15, 1904, under the command of Admiral Zinovy Petrovich Rozhestvensky . The fleet reached the China Sea in early May 1905 and made for Vladivostok. The Japanese under Admiral Heihachiro Togo intercepted it, May 27 to May 29, in Tsushima Strait, between Korea and Japan. The Japanese fleet, which was superior in speed and armament, sank, captured, or disabled eight battleships, nine cruisers, six special-service steamers, and several other ships in the Baltic Fleet. Some 4000 Russians were killed, and three admirals and 7300 sailors were captured. The Japanese lost three torpedo boats, 116 men killed, and 538 wounded.

After Port Arthur and the defeats at Shenyang and Tsushima, the emperor accepted the offer of mediation extended by U.S. President Theodore Roosevelt. The Japanese, who were victorious but financially exhausted, also agreed to negotiate. On September 5, 1905, the Treaty of Portsmouth was signed. Russia surrendered its lease to Liaoyang and Port Arthur, ceded the southern half of Sakhalin, evacuated Manchuria, and recognized Korea as a Japanese sphere of influence.