Claim: Theodore Roosevelt was President of the United States.

The Russo-Japanese War -LRB- , Russko-yaponskaya voina ; Nichirosensō ; 1904 -- 05 -RRB- was fought between the Russian Empire and the Empire of Japan over rival imperial ambitions in Manchuria and Korea . The major theatres of operations were the Liaodong Peninsula and Mukden in Southern Manchuria and the seas around Korea , Japan and the Yellow Sea .   Russia sought a warm-water port on the Pacific Ocean for its navy and for maritime trade . Vladivostok was operational only during the summer , whereas Port Arthur , a naval base in Liaodong Province leased to Russia by China , was operational all year . Since the end of the First Sino-Japanese War in 1895 , Japan feared Russian encroachment on its plans to create a sphere of influence in Korea and Manchuria . Russia had demonstrated an expansionist policy in the Siberian Far East from the reign of Ivan the Terrible in the 16th century . Seeing Russia as a rival , Japan offered to recognize Russian dominance in Manchuria in exchange for recognition of Korea as being within the Japanese sphere of influence . Russia refused and demanded Korea north of the 39th parallel to be a neutral buffer zone between Russia and Japan . The Japanese government perceived a Russian threat to its plans for expansion into Asia and chose to go to war . After negotiations broke down in 1904 , the Japanese Navy opened hostilities by attacking the Russian Eastern Fleet at Port Arthur , China , in a surprise attack .   Russia suffered multiple defeats by Japan , but Tsar Nicholas II was convinced that Russia would win and chose to remain engaged in the war ; at first , to await the outcomes of certain naval battles , and later to preserve the dignity of Russia by averting a `` humiliating peace '' . The war concluded with the Treaty of Portsmouth , mediated by US President Theodore Roosevelt . The complete victory of the Japanese military surprised world observers . The consequences transformed the balance of power in East Asia , resulting in a reassessment of Japan 's recent entry onto the world stage . It was the first major military victory in the modern era of an Asian power over a European one . Scholars continue to debate the historical significance of the war .