Threats against the United States. While we have bases in Japan, that's not a threat directly against the United States.
I do not equate this to the Syrian red line, but it probably wasn't well advised. The good thing is that "fire and fury" can mean damned near anything, the bad thing is everyone knows what it is supposed to mean. If anything that can be perceived as a threat is given, we will look as stupid as we did when we didn't respond previously, and we have a hell of a lot of Dixie Chicks ready to sing to anyone who will listen how stupid we look.