The Russo-Japanese War did not really start as the result of any particular incident. Instead, it started because of a relatively long-standing conflict between Russia and Japan over Manchuria and Korea.
Russia and Japan went to war in 1904 because each country had imperial ambitions in Northeast Asia. Russia wanted to have access to warm water ports, mainly that of Port Arthur on the Liaotung Peninsula that juts into the Yellow Sea. It had completed the Trans-Siberian Railway linking Russia with the Pacific and it wanted more control over that area. At the same time, Japan was becoming a powerful nation. It had become quite modernized and felt that it should be treated as an equal to the European powers. It felt that Manchuria and Korea should be within its sphere of influence.
Japan did not feel that it could evict Russia from the whole area. Therefore, it proposed that it should gain control of Korea and, in exchange, give Russia more control of Manchuria. When this was rejected, negotiations broke down. Japan decided that it would have to resort to war. It therefore broke off diplomatic relations with Russia on February 6, 1904 and attacked Port Arthur on February 8.