Citing "multiple senior U.S. intelligence officials," NBC News reports that the U.S. "is prepared to launch a preemptive strike with conventional weapons against North Korea should officials become convinced that North Korea is about to follow through with a nuclear weapons test."

Here's the problem — neatly summarized by the New York Times' national security correspondent David Sanger, who's covered North Korea and nuclear proliferation for three decades: North Korea has had — and still has — a non-nuclear way of destroying Seoul, one of the biggest and most prosperous capitals in Asia," Sanger told NPR's "Fresh Air."

Sanger vividly describes the scenario that's terrorized (and paralyzed) military planners since the 1990s:

"The capital of South Korea...It is closer to the border — or the northern border of the Demilitarized Zone than Baltimore is to Washington. So the problem has always been that with mortars that are hidden away in the mountains there, there could be such destruction rained across a city of 14 million people or so and of course one of the economic hubs of Asia, that no one really thought it was worth the risk to take out the theoretical threat of a nuclear North Korea."

Bottom line: "Every time we face this issue," Sanger says, "American presidents sort of backed down and started up a negotiation." Trump wants North Korea to know there's a new sheriff in town, but the same 20-year dilemma confronts him.