I respectfully disagree. The North could put 2 million men on the southern border of North Korea, and we would not act until they fire or move on us. We would only use conventional means which would limit a ability to effect millions. In that theatre we would not use nuclear devices, unless they first used it on our soldiers, and maybe not until they attacked our homeland. That means conventional warfare.
That would be good trick, because they only have one million men in uniform. And why would they ever put all their troops in one spot anway? U.S. forces would literally blacken the sky with large anti-personnel bombs, (MOABS, daisy cutters), cruise missiles, tomahawks, F-18 attacks, etc. And don't forget the ROK Army, their artillery, airforce, and their own rockets and missiles. If NK ever stood a million men directly on the border they'd be target practice.