I don't know if this will be changed in the LN, but in the WB version in a special chapter, After the end of the Loria-Qua-Toyne war, the Qua-Toyne leaders take the Japanese representatives to a sacred place of theirs that has one of the God emissaries weapons that was shot down, and that weapon is revealed to be a Japanese Zero fighter, so it basically confirms that at least some army group or a part of the navy from WW2 Japan was transported there, defeated the Demon Lord and its army, and then they were brought back to our world. And it's also revealed during the Japanese intervention of the Demon Invasion that will occur a little after, that the Demons are actually some kind of weapon/servants of the ancient magical empire, and that they are trying to prepare the land for their creators when they return