The north didn’t invade the south though, no Koreans agreed that the US supported parallel was a permanent division of the country, both North and South fully intended to create a united Korea. Tens of thousands of Koreans were already dead from purges and suppression of uprisings in the south when the operation started. It was literally an ongoing civil war that had momentarily frozen.
I’m not sure on what information you base this claim, but as far as I know the 38th parallel was agreed upon because both the udssr and the US wanted total occupation of Korea for themselves, but they both wanted to potentially avoid an armed conflict so tried a compromise.
Then the north korean part, supported by China and unofficially by the soviet union, invaded the south to establish total control.
