The rules exist to serve as an excuse to tell smaller countries what to do, to keep them small. They justify invasions, not prevent them. By controlling the UN we can tell Iraq what they can and can't do, invade them, sanction and bomb them weekly for over a decade, and then invade them again, amounting to a death toll, directly or indirectly, in the millions, all without any consequence. Meanwhile North Korea shoots a crappy missile out into the sea and its sanction time again.
It was never designed that way, but in reality it works that way. What's hilarious is to hear ultra-concervative Americans talk about how we should leave the UN because it holds us back, neuters our sovereignty. In fact, its one of our most powerful weapons!
The winners write history. The very tactics that won WWII are now banned by the UN and the Geneva convention (ie: massive and sustained carpet bombing of civilian targets). If the axis powers had prevailed the same rules would exist, only it would have been Churchill and Truman who would have been tried for the same warcrimes we eventually charged Nazis with.
