According to Bob Woodward they've been on that line at least once that he's written about.
Pres. Trump wanted to tweet that he was pulling ALL military families out of SK and they had to break it down piece-by-piece on why that was a BAD idea.
"Why would that be a bad idea?"
"Mr. President, if you tweet that,
NK would take that as an American attack on them would be imminent and they would more than likely attack the South."
"So, we'd just attack back and win."
"Yes, we would sir, but that would bring in China also and more than likely kick off WWIII at the cost of millions of lives."