You are unaware of why the north attacked the south. The south was sending assassins into the north to kill all the partisans that had opposed the Japanese they could find. Syngman Rhee and his pals were basically puppets of the Japanese during the occupation. Also, the US did not station enough troops in the south to defend it. Like most Americans, you seem to know little of the history of Korea before Theodore Roosevelt said "The Japanese should have Korea", and moved in and set up a colony. Prior to the Japanese takeover, about 80% of Koreans were basically serfs to the aristocracy. The Korean aristocracy that resisted were killed by the Japanese, but those who collaborated became their flunkies. They were the nucleus of the government that was set up in the South after WWII. The point is that it was NOT a struggle between ourt good guys and their bad guys.
South Korea was certainly NOT inoffensive in the late 40's and early 1950's.
The US is not going to attack North Korea, and any attacks that the North engages in will be limited.