What started the Russo-Japanese War?
Japan and Russia's territorial disputes over China and Korea
The Russo-Japanese War (1904-1905) occurred because the Japanese and Russian Empires had territorial disputes over parts of Manchuria and Korea.
Russia desired a warm-water port, which would be beneficial for trade, since most of Russian is permanently frozen. Meanwhile, the Japanese Empire wanted to expand into Korea and China.
Japan won the Russo-Japanese War, and this was the first time that an "Asian" power defeated a "European" power.
There was another almost war between the Japanese and Russians in 1938-1939 also over the resources of Manchuria
The Japanese having won the war of 1905 had taken over Korea and moved into Manchuria. In 1938 the Japanese trying to extend their influence in the far east attacked territory that threaten the Russia port of Vladivostok . The Japanese assault was thrown back. Later in 1939 Mongolian troops sponsored by Russia attacked Manchuria but were repulsed by the Japanese. The Russians counter attacked destroying a Japanese regiment. All total the Russia lost 10,000 men and the Japanese 13,000 men.
Despite the apparent Russia victory a truce was declared and Japan retained control of the disputed Manchurian territory that triggered the conflict.