Interesting, so what you're saying is after the Japanese lost their Korean colony, the country was split into two, then communist aggression from the North invaded the South which resulted in foreign powers stepping into defend the Southern half of the country.
Sounds pretty similar to France losing their Vietnamese colony, and the country being split into two, and then a foreign power coming in to defend the southern half of the country.
So it looks like we agree?
At no point during the entire Korean War was there a Japanese colony anywhere on the Korean peninsula. Or, for that matter, anywhere on earth.
If you're arguing the South Korean government whose sovereignty the UN went to war to defend, was somehow "aligned" with their former occupier Japan which was itself then occupied by the US after its unconditional surrender to end WW2... sorry, you're just woefully ill informed.
There are dozens of good books on this topic.