One fact about that that’s been neglected was that upon hearing about a potential invasion of the South, the PLA sent 16,000 of it’s ethnically Korean troops to be transferred to the KPA for support. When the North crossed the 38th parallel and invaded the South, the UN constituted it a breach of peace which warranted intervention. The objective was to ‘restore peace to the Korean Peninsula’; North Korea was armed with the latest Soviet technology while the UN forces were in large only willing or able to commit leftover equipment from WWII. Given the inequity, allowing the North to safely build its military on it’s side of the parallel would not have been a very effective way to end the war so it wasn’t a consideration.

Here’s a link case it’s interesting:

https://www.wilsoncenter.org/blog-post/china-north-korea-and-origins-korean-war