The United States does not have any business screwing around with treaties and alliances, as well as the defense of foreign nations. We need something that I've discussed in an earlier post, that is, we need to pursue a policy of non-interventionism. Non-interventionism is not the same as isolationism, so don't get the two confused. I can go over the difference some other time.
In short, the United States needs to sever all current alliances (UN, NATO, NAFTA, etc.) and remove troops from foreign countries so that they can play a defensive, not offensive role. With no alliances, there can be no enemies, so the US could open trade to any country in the world; North Korea is no exception. If they want to trade with us, they can. If they don't want to, they don't have to. History is shown however that trade brings peace, so that might be the best course of action on behalf of the North Koreans (whose GDP is only 28-40 billion compared to our 14 trillion). It could be said that the same is happening in China. We don't attempt to invade China, instead we maintain relations and trade with them. Lo and behold, they are slowly becoming more capitalistic and friendly with us. Remember, the Soviet Union did not fall because we attacked it. It fell because communism is doomed to fail. Eventually, communism in North Korea will fail on its own - we don't need to force it by invading, we only need to trade.
South Korea can defend itself if need be. Their GDP is nearly 100 times that of North Korea's, and can definitely field an effective military if need be. If North Korea attacks the South, then the South has the right to retaliate. The US on the other hand does not have the right to assault the North, the fight between the two is none of our business.
So, China and North Korea are closely allied. Is it possible that a war between the North and South could draw all of this big world powers (US, China, Russia, Europe) into a massive war? I don't think it's likely. First off, China is not going to attack its largest trade partner; what would be the point? Secondly, North Korea has no air force or navy to attack us with. Hell, they can't even feed their own people.
What we should worry about is the domino effect. That is, if North and South Korea go to war, everyone they are allied with is obligated to follow suit. This can create a lot of problems, unless we remove alliances. Then we wouldn't have to worry about it.
Unfortunately, the US is going to get dragged into this conflict further because there is no one making sensible foreign policy decisions in our government. Go America!