USA afraid of communisms spread. Sends MacArthur to Korea. He crushes the Korean communists. USA believes it to be a clean sweep. Mao surprise s USA with massive army encirclement of USA forces. USA forced to flee south and hold the line that became demarcation between north and south.
There is a 0% chance China would allow the USA to take NK.
They also love provoking Japan. Their entire origin story is about fighting against the Japanese in the 30s.