There was no mutual antagonism. Japan of this period was expressly a violent, imperialist project, from its annexation of parts of China and colonizing of South Korea following the Russo-Japanese war to later invasions of modern day Taiwan and the Philippines, all of whom were ill-equipped to resist them. The Mongols tried to invade and China and Japan fought over the Korean peninsula sporadically during what we call the Middle Ages, but that's about the extent of prior regional strife.