They can hold them up. North has over 1,000,000 regulars and their reservists aren't quickly mobilized. South has 700,000 regulars and 3,000,000 reserves at readiness. The South have been effectively holding them back for decades.
Problem is that Obama wants re-election so won't get involved. Probably pull everone out of South Korea. That removes another barrier to the North using their nukes on their long range Nodong 1 missiles. They only have eight so they'll want to target carefully.
Remember they've got loads of chemical weapons, over 5,000 tons of them. Yes, this will be a WMD war before the end of the week.