The Japanese came from Koreans.

[CarbonX13]

It's a language's structure that tells the tale of its origin. Languages are much more reluctant to alter structure than they are to borrow words. Chinese is structurally very very different from both Japanese and Korean, although both languages borrow heavily lexically. Korean and Japanese undoubtedly have too many structural similarities for them to be unrelated.

Very true. My point was, the Korean Peninsula had such little political impact at the time that there was really no way their local dialect would be able to impact the neighbouring empires of China and Japan. The people in the mainland Asian region migrated to Japan, likely through the Korean Peninsula, but that doesn't indicate that their language heritage comes strictly from that region. For all we know, "Korea" was just a small bunch of colonies or an insignificant kingdom when the Chinese and Japanese empires were flourishing. Stating blatantly that Japanese language comes from Korea is a hyperbole of the truth if anything. The Koreans' ancestors didn't just appear there, they must've migrated from somewhere further inland. A more reasonable theory would be that the people there were speaking a 'lost' dialect of Chinese or another Sino language, and that the Korean and Japanese languages of today derive from that.

We should also keep in mind that even more similar linguistic characteristics may have developed in the more recent past, as Korea was a colony of the Japanese empire for a few decades in the century past, and was even more influenced by their power-hungry Japanese neighbours during the peak of their empire expansion.