So whats the deal? we have to wait until North Korea drops a nuclear warhead on South before we can retaliate?

Minor skirmishes have broken out between the two countries ever since the armistice. This is nothing new although it remains to be seen of course whether it escalates. Just a few months ago North Korea sunk a South Korean ship with a torpedo. Many people died. South Korea chose not to retaliate for their own reasons. The only reason we haven't intervened is because the South Koreans don't want us to do so yet.