With North Korea threatening to send a salvo of ballistic missiles close to a U.S. military hub in the Pacific, pressure could grow for Washington to put its multibillion-dollar missile defense system into use and shoot them out of the air before they can pose a real threat.

But should it? Could it?

That's no easy call.

North Korea claims it is in the final stages of preparing a plan to launch four intermediate-range ballistic missiles over Japan and into waters just off the island of Guam, where about 7,000 U.S. troops are based.