In 1894-5 Japan defeated China in the First Sino-Japanese War. With that victory the Japanese had hoped that Korea, a territory that had been giving tribute to China for centuries, would fall under it's control. It didn't, because Imperial Russia had military and economic interests in Korea that it was willing to go to war to defend. So, in 1904-1905, Japan and Russia went to war, partly over the question of who would be the dominant power in Korea. To the shock of almost everyone except the Japanese, Russia was defeated, and in the Treaty of Portsmouth which ended the war and which won American president Theodore Roosevelt the Nobel Peace Prize for his role in sponsoring and hammering out the treaty, Japan was recognized as having control over Korea, though no one asked the Koreans about it.