We already have "defense treaties" with Japan and South Korea. What you are talking about is more like a war pact.
Having spent 3 years in the US military working with both Japan and South Korea operating against North Korea, I can tell you America and its allies are more than capable of an assault on that poor nation.
I cannot agree with the idea of preemptive war. It is evil. I do not mean to mitigate your position, but unless your "class" was of the "military A or C school" kind, I have doubts as to your knowledge of the fundamentals of war.
Just because we can do something, does not mean we should. Kim has been sabre rattling for a long time.
Personally, I think North Korea (or South Korea) should have assassinated him long ago. They also should have assassinated his father.
If the Koreans don't mind Kim and his horrible government, if they don't mind all the evil shit he's done, all the innocent people he's kidnapped and killed, if they don't mind the split Korea, and the starvation of millions enough to do something, seriously, America should stay out of it.
Of course the moment that bastard sends a missile right towards Hawaii we should send rain of fire upon the whole of his empire. But we won't. We will not touch Kim. I do not know why. North Korea wouldn't have China's and Russia's backing like they did in the war, North Korea would fall in a day if we actually tried.
I certainly don't think preemptive war will go away, just as it was our excuse in Iraq, it will be again, but it is always wrong. In this instance, Kim and whoever takes his place is untouchable.