Because Japan and South Korea could be in danger of missiles or even a atomic bomb nuke if N korea has them.
Japan and South Korea are our aliens and japan also makes Anime and is the country that creates the best games, it takes the whole west to make more good games than japan but no contry alone can match them.
Also China doesn't want to protect North Korea anymore. The only reason to not invade north Korea is because we cannot defeat China in Asia, but since China is tired of them now it is our chancew
The big problem with North Korea is Russia and China. Both are allies with NK and share a border, both do not want a US ally next door.
Now with China thankfully they are quite close with the US and I think they like the financial benefits of world wide trade than the trade they do with NK.
Russia however I am not so sure, and who knows what Putin will do if the US did invade NK.
The North Korea problem is not one so easily solved.
Russia is nothing anymore @Babbit55 their economiy is the size of italy.... they act big but they are much smaller that thet pretend to be. USA or China would stomp them and Germany, Japan and the UK could beat them if they invested more on military ---