Respuesta :
Japan entered World War I on the Allied Forces' side because it had a previous treaty of alliance established with England. The correct option among all the options that are given in the question is the first option or option "A". Another reason might have been that Japan thought that since the Germans were busy in Europe, they could grab their colonies easily.
Japan enter World War I on the Allied Forces' side, because It had a previous treaty of alliance established with England.