There are are a lot of misconceptions here. I will try to address some of them.
1. The UN is not a country and it is not a policeman. It is a forum for countries to interact. A forum cannot, by definition, be "issed". For the purpose of this discussion it is more useful to think of it as a collection of buildings in which ambassadors and representatives from each country speak with one another, rather than a body with its own interests.
I will explain a little more about its structure. It is composed of 2 main bodies: the Security Council and the General Assemmbly.
The Security Council is a powerless, symbolic body composed of the most powerful countries in the world. It has the 'ower' to authorise the use of force against countries, but in the real sense it is powerless because the exercise of that power depends on its members agreeing with one another. More specifically, 5 of its members--Russia, the US, the UK, France and China--must all vote in favour of (or abstain from) a Security Council Resolution for it to pass. Once it passes, it is said to be binding in international law. However, because no country enforces international law unless it is in its interest to do so in the particular circumstances involved, really the resolution is of no effect. The General Assembly is much the same, except all UN member countries may vote, no country has a veto and the resolutions aren't binding. In short the UN as a body is toothless. It has symbolic influence over world politics. Any action it takes--whether it be sanctions, or a peacekeeping mission--is in truth is the 'action' of the countries that bothered to go through with it, because they thought it was in their interests to do so.
2. Because the UN is simply a forum, "UN Backing" or "UN approval" for a given action (including military action) simply means that the member states of the UN's interests align with that of the country proposing the action. This does not mean the action is (1) morally justified or (2) politically necessary. It simply means that some other country thinks it either morally justified or politically necessary. Why the United States should look to the Communist Party of China for approval before undertaking any given action is beyond me. By saying "we need UN backing before doing X", we are really saying "I'd like the tyrants of China to decide that they would benefit from my actions before I undertake them."
Of course I do not suggest that cooperation is bad. The US should always do its best to cooperate in undertaking any given course of action--especially with its fellow allied democratic countries. And if the approval of a tyranny like China can be obtained at little price, then all the better. But as a general rule it is quite unnecessary.
3. However, I should add that one should one country should never invade another unless it is reasonably certain that it is capable of victory. That could mean entering into a war with allies, such as France, the UK, etc. Obtaining their backing and support is always useful, particularly because the US lacks the resources to protect its interests (and those of other allied countries) in all cases.
4. No one--NO ONE, in America, in Sweden, or in Transylvannia for that matter-- is thinking about invading or attacking North Only the North Koreans themselves think otherwise. This is because they are a collection of paranoids who have lived their lives closed off from the rest of the world.
5. What is possible is a heightened sanctions effort against North Korean shipping and trade. This may depend on the cooperation of neighbouring countries--e.g. China, Japan, etc. Given that China is North Korea's principal sponsor, I don't think Chinese cooperation is likely.
6. North Korea is unlikely to attack anyone, because it is unlikely to win. The reason it builds up weapons stockpiles and tests weapons is to remind people that is is a dangerous country that ought to be propped up. North Korea's people are starving and its economy constantly teeters on collapse. It needs money and food, and intimidation is a good way to get it, because no one wants to go to war with North Korea.
7. Use of atomic weapons is not banned, but it is very unlikely to happen, both because it is deplorable and also because it could lead to mutually assured destruction (MAD). The "MAD" doctrine applies where both the attacker and defender have nuclear weapons. The logic is obvious: you nuke me, I nuke you back--and both of our nations become no more than piles of ash. This explains why there have been no nuclear wars since World War II.
8. North Korea is a small country with an immense army. But the size of its army or whether victory against in battle is not to the point (US victory against it is assured). More importantly, it is the consequences that follow from invasion that are to be feared. Firstly, NK possesses nuclear weapons. Invading it with the intention of overthrowing the NK Communist Party will greatly increase the chance that those weapons will be used--against South Korea, for example. No one wants nuclear weapons to hit the ground ever again, and for this reason no one wants to risk invading North Korea. Secondly, invading North Korea will mean that the country will be in chaos. Millions of its people will run to the Chinese border, or starve, or both. This is bad in and of itself, but needless to say that it is something China and South Korea do not want. Thirdly, the North Koreans will be capable of bombing South Korean cities and causing many deaths. This is very undesirable. Fourthly, the United States may sustain casualties of its own in any war effort. Also undesirable. This is simply scratching the surface of the list of why invading or attacking NK is a bad thing.
If this sort of thing is of concern to you or troubles you, I suggest you do some more reading on the topic of foreign policy.
Books--
History books can give an insight as to how governments think about foreign policy, and also give an excellent insight into how the world came to be as it is today. English history is always interesting, as it documents how country went from being an absolute monarchy to a parliamentary democracy over the course of centuries. In the process the Brits did many things--many very bad, but also some good things as well. It makes for interesting reading.
Ideology--
To get a good grounding of foreign policy it is necessary to understand the various belief systems that people rely on to form governments and societies.
To understand North Korea, for example, I would recommend you read:
- Some of Marx's writings (the Communist Manifesto is a good start).
- "Nomenklatura", a book on the process by which the Russian Communist Party turned from naive ideologues to vicious murderers. This book is of immense significance because it explains the structure of government not just in the defunct Soviet Union, but also in North Korea, China, Syria, Iraq (formerly), Egypt, and many other countries where socialist dicatorships of a similar kind arose. It gives an excellent general overview of their interests.
Magazines--
Foreign Affairs, Foreign Policy and the National Interest are American foreign policy publications. They give an excellent insight into how American foreign policy makers and academics think about American foreign policy, and they are very influential for this reason (because they're written by the people who make foreing policy in America. If you want to know what members of the government think about foreign policy this is a great start.
