[ninty]

so how can it enforce anything?

Here's the kicker. If you want the UN to enforce things, you must give it power. This results in your own country losing power. Do people want to give up Sovereignty to the UN?

If the UN is to have an actual army, then they would need a governing body to make decisions. The only real way to do this is with a one world government, which i'm not too fond of.