@phil - best not to ask why they is threatening war, or why they feel the need to say these things, the answer is watch Fox News then kill, kill, kill.
In this situation if you were president of the USA would you do the following:-
1) Threaten to meet their use of weapons with bigger weapons
2) Seek dialogue through China/UN to calm the situation
3) Get in a preemptive strike
4) Go on a diplomatic offensive to reassure North Korea you have absolutely no interest in invasion, get Obama on the telly saying he is looking at ways the two countries can cooperate with each other to build bridges, trust, sporting & cultural ties
5) Meet their war rhetoric with increased war rhetoric of your own, mobilisation of weaponry, troops, reconnaissance activities, start deploying missile defence systems, etc, etc.
There are always choices, which one would you choose, is it the same as the one the US Govt are choosing, I think I'd be talking the situation down if I were Pres.