From the story here:
Last month, former Secretary of Defense Ash Carter said about an attack on North Korea, “We’ve always had all options on the table… I wouldn’t take any off.” Were North Korea to commence a massive artillery barrage on Seoul, and especially if they used chemical or biological weapons, one potential U.S. response could be nuclear retaliation against Pyongyang and/or their artillery at the border. (Perhaps dialing down the “variable yield” of our B-61 nuclear bombs to reduce radioactive fallout and civilian casualties). As outlandish as this response may seem right now, some U.S. war planners do not think so; it could be considered “proportional” and therefore within the laws of armed conflict. If a nuclear bombing were ordered under Obama–who was widely perceived as thoughtful and hesitant to use force—the world may have been largely willing to listen to his logic. Unfortunately, because much of the international community views President Trump as reactive, even unstable, any such U.S.-initiated action would likely be met by ferocious global condemnation. This perception of our President may be a tactical consideration for our war planners led by the very talented head of Pacific Command, Adm. Harry Harris.