North Korean Juche must have been a one, big, fat irony since it is based on a kind of fascist system that their former Japanese overlords used. Though there is the issue of Korean artifacts that the Japanese government hasn't yet returned to South Korea. Then there's also the Japanese claim that Japan and Korea were of the same nation due to the legend that a princess or queen ended up in Japan after she fled from Korea, a legend that I don't know well.
Yeah, that was pretty ironic. I also addressed the artifact issue earlier, and it's probably not something that can be resolved soon.
Regarding the relationship between Korea and Japan, the historiography from both sides tends to get complicated, of which many components were fabricated for various reasons. The
Samguk Yusa claims that a Silla fisherman supposedly got lost and landed on Japan, after which he and his wife were eventually proclaimed as the first king and queen, although this has never been historically verified. On the other hand, Japan began to interpret passages from the
Nihon Shoki as basis for the fact that
Jingu supposedly "conquered" Korea (which was divided among five major entities at the time), although no archeological evidence has been found, and there are no corresponding details within Korean sources. In addition, the
Kojiki does not mention any similar events, and there is virtually no way to verify the chronology of the early Japanese rulers, many of whom may have been fabricated. Another theory proposed that the Japanese rulers were originally located on the Korean peninsula, but the Koreans eventually revolted and established their own rulers, while the deposed ruler was forced to flee to Japan, suggesting that the Koreans were subservient. These theories, along with others, such as one implying that Gojoseon originated as a Chinese colony, were used to justify the Imjin War and the Japanese colonization of Korea, although none of them have been historically verified.
In other words, Japan took pains to fabricate historical justification for rule over Korea, many of which were incorporated by the historical committee that I mentioned earlier. However, various historical and archeological evidence all suggest that for at least four centuries, Korea transmitted culture and technology to Japan, many of which originated from China, although some were later imported directly from China, not to mention immigration and widespread trading/diplomatic contacts among the countries involved.