What Will The USA Do To S KOREA?

Most likely drop bombs on it, as they have in other countries, that were perceived as a threat to the United States. South Korea has always been considered a rogue state by America and its Government officials.
The United States has been trying to negotiate with South Korea, to give up developing nuclear capabilities and other military weaponry. The reason for this is, that America considers the government in place to be unstable and unpredictable, in much the same way as they view Iran.
With whom they have been trying to negotiate for some time,
to give up developing nuclear capacity. They have refused.
This situation is of great concern to America and it's allies.
They have at present, reached a stalemate.

United States would not drop bombs on S Korea.  You are thinking N Korea.