The Dragon Swallowed the Sun
This is a continuation of the Hail to the Qing achievement.
Now that Japan is pushed out of Asia, research and build marines, naval transports and submarines and place them on the Korean port. Increase Relations with the United States and take the Train Marines Focus. This will give more special units and a bonus to marine research. Find out how many ships the Japanese have on the Sea of Japan. This is done by hovering over it and checking the tooltip. Build more submarines than that. Japan will go to war with the Allies allowing Manchukuo to join their faction. Do not do it because it will allow them to enter territory under Manchukuo giving them more war score against the Japanese. Naval invade Japan when ready. Either take the closest province near Hiroshima, or take the port north of Osaka. The port may or may not be guarded. If you take the Province near Hiroshima, Hiroshima may be guarded. Take Nagasaki if there are too many Japanese divisions. Once you take a port, increase its level so more divisions can be fielded in Japan. Tokyo and everything west of that might be enough to capitulate them. Once Japan capitulates, Annex all of their territory. The achievement may not fire if Japan gets puppeted by someone else: if the Allies or Soviet have a large contribution, you can pre-empetively puppet Japan and annex it through the autonomy system.