Why was Theodore Roosevelt worried about a war between Russia and Japan?

The Russo-Japanese war started in 1904 and ended in 1905. It was fought between Japanese and Russian empires due to the claims over Manchuria and Korea. It fighting was confined in Liadoing peninsula, the sea around Korea and Japan and Southern Manchuria. The fighting started when Japan attacked Russian fleet at Port Arthur without giving any formal declaration. It caused huge casualities on both sides. Japanese urged president Roosevelt to negotiate peace agreements. The representative of both the countries met at Portsmouth, New Hampshire in 1905. Roosevelt wanted to end the war with such terms that both Russia and Japan could play an important role in Northeast China. He was worried because if Japan managed to drive Russia out entirely then it could harm American Interests in that region.