At the very least South Korea needs to be involved. A strike against North Korea will put South Korea immediately into an expensive all out war even if they didn't start it. Second, even if the US military was "still" mighty[1] we would need the blessing of Russia and China or at least their promise not to retaliate and start World War III.
To the person who said...
> There are also credible reports of genocide. That'd be reason enough don't you think?
NO! Many times no! This is one of the reasons the world hates us. We think it is our position to unilaterally police everyone in the world. Everyone should universally decry genocide, absolutely and aggressively. But we are citizens of the world not dictators. We should not be pre-emptively striking. We should not be sending money and weapons to other countries (or at least only send them to one side of the fight for god sake so we don't have two sides fighting each-other with our own weapons!). Every time we have been interventionists either overtly or covertly it has caused unintended consequences and often disastrous ones. When will we learn that it hurts more than helps?
* I say this as a born American.
[1] We spent more on our military than any other country by a huge margin. I really don't understand