From what I gather, NK is talking about launching missiles (nuclear tipped or not) towards Guam but not to hit Guam. They will land 15 to 25 miles away from Guam. To me this is still unacceptable and should be answered with an overwhelming response. If America is not to be messed with, then America needs to make it perfectly clear, whether it's popular with the rest of the world or not, that you DO NOT threaten it in any way what so ever. Other countries will get the message and we can regain our standing and credibility.
The chances are, If NK launched missiles toward Guam, we would never know if they would have hit the island, or landed within 25 miles. Japan has said that if missiles were launched toward Guam, they would shoot them down, since they would travel over Japan on the way to Guam. Also our own military would be attempting to shoot them down. So unless we were able to retrieve the destroyed missiles from the ocean floor, we might not even ever know if they were nuclear tipped.