China stated that if NK attacked Guam, then China would remain neutral.  Russia has remained vague about its intentions.

Kim's continual breach of international law and UN resolutions has placed China & Russia in a difficult position with respect to their soi-disant ally.  As much as they like to see NK threaten the US, they understand that Kim's reckless provocations which include using thermonuclear weapons against Australia, Japan, and the US may bring a justified retaliation.

Xi and Putin may well reckon that it is not worth risking Beijing and Moscow to defend a pathological certifiable lunatic with WMDs whom most of the world hates.

Mattis and Durban along with McMaster and Kelly have a reputation for being competent.  If there is "shock & awe,"
then it should be more successful than the Iraq iteration.  

Nevertheless that would be the very bad option with only allowing Kim to continue with his WMD/missile proliferation and incessant threats being worse.