I don't see any reason why the Americans would join in the Russo-Japanese War. The Americans had always been friendly towards the Russians up until Stalin and the Americans had a 'fair' relationship with Japan at that time. So the best thing the Americans would do is to be neutral like they did in OTL.
If there is another power intervening, it would probably Britain for the Japanese. But since the Japanese won the war in OTL, British involvement on the Japanese side would basically be overkill.