The Russo–Japanese War (日露戦争 Nichi-Ro Sensō, Русско-японская война Russko-Yaponskaya Voyna, 日俄戰爭　Rìézhànzhēng, February 10, 1904–September 5, 1905) was a conflict that grew out of the rival imperialist ambitions of the Russian Empire and the Japanese Empire over Manchuria and Korea. The major theatres of operations were Southern Manchuria, specifically the area around the Liaodong Peninsula and Mukden, and the seas around Korea, Japan, and the Yellow Sea.

The Russians were in constant pursuit of a warm water port on the Pacific Ocean, for their navy as well as for maritime trade. The recently established Pacific seaport of Vladivostok was the only active Russian port that was reasonably operational during the summer season; but Port Arthur would be operational all year. Negotiations between the Tsar's government and Japan between the end of the First Sino-Japanese War and 1903, had proved futile. The Japanese chose war to maintain exclusive dominance in Korea.

The resulting campaigns, in which the fledgling Japanese military consistently attained victory over the Russian forces arrayed against them, were unexpected by world observers. These victories, as time transpired, would dramatically transform the balance of power in East Asia, resulting in a sober reassessment of Japan's recent entry onto the world stage. The embarrassing string of defeats increased dissatisfaction of the Russian populace with the inefficient and corrupt Tsarist government, and was a major cause of the Russian Revolution of 1905.

The French financed half of the cost of the war, by their subscription to so-called Russian bonds ; post-USSR Russia reimbursed roughly 1.54 % of the capital to the French Republic without any interest, in 1996.